Azure AI Inference SDK - Beta 2 updates

[dargilco]

The main reason for this release, shortly after the first release:

• Add strongly-typed model as an optional input argument to the complete method of ChatCompletionsClient. This is required for a high-visiblity project. For this project, developers must set model.

Breaking change (noted in CHANGELOG.md):

• The field input_tokens was removed from class EmbeddingsUsage, as this was never defined in the
  REST API and the service never returned this value.

Other changes in this release:

• Addressing some test dept (work in progress)
  • Add tests for setting model_extras for sync and async clients. Make sure the additional parameters appear at the root of the JSON request payload, and make sure the unknown_parameters HTTP request header was set to pass_through.
  • Add tests to validate serialization of a dummy chat completion request that includes all type of input objects. This is a regression test (no service response needed), as the test looks at the JSON request payload and compared to a hard-coded expected string, that was previously verified by hand. This test includes the new model argument, as well as all other arguments defined by the REST API. It will catch any regressions in hand-written code.
• Update ref docs to remove mentioning of the old extras input argument to chat completions in hand-written code. The name was changed to model_extras before the first release, but looks like we still had some left-over ref-doc comments that describe the no-longer-existing argument.
• Remove unused function from the sample sample_chat_completions_with_image_data.py. Forgot to do that in the first release.
• Minor changes to root README.md
• Indicate that complete method with stream=True returns Iterable[StreamingChatCompletionsUpdate] for
  the synchronous ChatComletionsClient, and Iterable[StreamingChatCompletionsUpdate] for the asynchronous
  ChatCompletionsClient. Per feedback from Anna T.
• Update environment variable names used by sample code and test to start with "AZURE_AI" as common elsewhere, per feedback from Rob C.

[azure-sdk]

API change check

API changes are not detected in this pull request.