Add value() and name() methods to enum types. (#1773)

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:
```
synthtool > Executing /tmpfs/src/git/autosynth/working_repo/Dialogflow/synth.py.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:854131ec1af7b3a313253474c24748dc0acd217a58a0b74dbfb559f340a15d78
Status: Image is up to date for googleapis/artman:latest
synthtool > Cloning googleapis.
synthtool > Running generator for google/cloud/dialogflow/artman_dialogflow_v2.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/php/google-cloud-dialogflow-v2.
synthtool > Replaced 'Copyright \\d{4}' in src/V2/Gapic/SessionEntityTypesGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V2/Gapic/AgentsGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V2/Gapic/IntentsGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V2/Gapic/EntityTypesGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V2/Gapic/SessionsGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V2/Gapic/ContextsGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V2/AgentsClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V2/ContextsClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V2/EntityTypesClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V2/IntentsClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V2/SessionEntityTypesClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V2/SessionsClient.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/V2/AgentsClientTest.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/V2/SessionEntityTypesClientTest.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/V2/SessionsClientTest.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/V2/ContextsClientTest.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/V2/EntityTypesClientTest.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/V2/IntentsClientTest.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Intent_Message_Platform.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Intent_Message.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Intent_TrainingPhrase_Part.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Intent_Message_QuickReplies.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Intent_Message_ListSelect.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Intent_Message_SimpleResponse.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Intent_Message_LinkOutSuggestion.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/StreamingRecognitionResult_MessageType.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Intent_FollowupIntentInfo.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Intent_Message_SimpleResponses.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Intent_Parameter.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/EntityType_Entity.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Intent_Message_CarouselSelect_Item.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/EntityType_AutoExpansionMode.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Intent_Message_Image.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/SessionEntityType_EntityOverrideMode.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Intent_Message_Card.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Intent_Message_Suggestion.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Intent_Message_BasicCard_Button_OpenUriAction.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Intent_Message_BasicCard_Button.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Intent_Message_Card_Button.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Intent_WebhookState.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/EntityType_Kind.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Intent_Message_CarouselSelect.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Intent_Message_Text.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Intent_Message_BasicCard.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Agent_MatchMode.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Intent_Message_SelectItemInfo.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Intent_TrainingPhrase.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Intent_Message_ListSelect_Item.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Intent_Message_Suggestions.php.
synthtool > Replaced 'will be removed in the next major release' in src/V2/Intent_TrainingPhrase_Type.php.
synthtool > Replaced '(\\@type|\\@param) EntityType_' in src/V2/Gapic/EntityTypesGapicClient.php.
synthtool > Replaced 'EntityType_' in src/V2/Gapic/EntityTypesGapicClient.php.
synthtool > Cleaned up 1 temporary directories.
synthtool > Wrote metadata to synth.metadata.

```