feat: Add DeadLetterPolicy support.

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:

synthtool > Executing /tmpfs/src/git/autosynth/working_repo/PubSub/synth.py.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
0.29.1: Pulling from googleapis/artman
Digest: sha256:b2a73f4dda03ef8fcaa973e3ba26d0cf34091f6c22c70add663af325931aef4d
Status: Image is up to date for googleapis/artman:0.29.1
synthtool > Cloning googleapis.
synthtool > Running generator for google/pubsub/artman_pubsub.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/php/google-cloud-pubsub-v1.
synthtool > Replaced "'serviceAddress' =>" in src/V1/Gapic/SubscriberGapicClient.php.
synthtool > Replaced "'serviceAddress' =>" in src/V1/Gapic/PublisherGapicClient.php.
synthtool > Replaced '@type string \\$serviceAddress\\n\\s+\\*\\s+The address' in src/V1/Gapic/SubscriberGapicClient.php.
synthtool > Replaced '@type string \\$serviceAddress\\n\\s+\\*\\s+The address' in src/V1/Gapic/PublisherGapicClient.php.
synthtool > Replaced '\\$transportConfig, and any \\$serviceAddress' in src/V1/Gapic/SubscriberGapicClient.php.
synthtool > Replaced '\\$transportConfig, and any \\$serviceAddress' in src/V1/Gapic/PublisherGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V1/Gapic/SubscriberGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V1/Gapic/PublisherGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V1/PublisherClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V1/PublisherGrpcClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V1/SubscriberClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V1/SubscriberGrpcClient.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/System/V1/PublisherSmokeTest.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/V1/SubscriberClientTest.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/V1/PublisherClientTest.php.
synthtool > No replacements made in src/**/*.php for pattern <a href="/pubsub/docs/, maybe replacement is not longer needed?
synthtool > Cleaned up 1 temporary directories.
synthtool > Wrote metadata to synth.metadata.

[codecov]

Codecov Report

Merging #2260 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##             master   #2260   +/-   ##
========================================
  Coverage      92.6%   92.6%           
  Complexity     4467    4467           
========================================
  Files           307     307           
  Lines         13323   13323           
========================================
  Hits          12338   12338           
  Misses          985     985

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