feat: Add support for apiEndpoint and deprecate serviceAddress client configuration option.

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:

synthtool > Executing /tmpfs/src/git/autosynth/working_repo/Spanner/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/spanner/artman_spanner.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/php/google-cloud-spanner-v1.
synthtool > Replaced "'serviceAddress' =>" in src/Admin/Instance/V1/Gapic/InstanceAdminGapicClient.php.
synthtool > Replaced "'serviceAddress' =>" in src/Admin/Database/V1/Gapic/DatabaseAdminGapicClient.php.
synthtool > Replaced "'serviceAddress' =>" in src/V1/Gapic/SpannerGapicClient.php.
synthtool > Replaced '@type string \\$serviceAddress' in src/Admin/Instance/V1/Gapic/InstanceAdminGapicClient.php.
synthtool > Replaced '@type string \\$serviceAddress' in src/Admin/Database/V1/Gapic/DatabaseAdminGapicClient.php.
synthtool > Replaced '@type string \\$serviceAddress' in src/V1/Gapic/SpannerGapicClient.php.
synthtool > Replaced '\\$transportConfig, and any \\$serviceAddress' in src/Admin/Instance/V1/Gapic/InstanceAdminGapicClient.php.
synthtool > Replaced '\\$transportConfig, and any \\$serviceAddress' in src/Admin/Database/V1/Gapic/DatabaseAdminGapicClient.php.
synthtool > Replaced '\\$transportConfig, and any \\$serviceAddress' in src/V1/Gapic/SpannerGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/Admin/Instance/V1/Gapic/InstanceAdminGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/Admin/Database/V1/Gapic/DatabaseAdminGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V1/Gapic/SpannerGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/V1/SpannerClientTest.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/V1/SpannerClientPartialVeneerTest.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/Admin/Instance/V1/InstanceAdminClientTest.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/Admin/Database/V1/DatabaseAdminClientTest.php.
synthtool > Running generator for google/spanner/admin/database/artman_spanner_admin_database.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/php/google-cloud-spanner-admin-database-v1.
synthtool > Replaced 'Copyright \\d{4}' in src/Admin/Instance/V1/Gapic/InstanceAdminGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/Admin/Database/V1/Gapic/DatabaseAdminGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V1/Gapic/SpannerGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/Admin/Database/V1/DatabaseAdminClientTest.php.
synthtool > Running generator for google/spanner/admin/instance/artman_spanner_admin_instance.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/php/google-cloud-spanner-admin-instance-v1.
synthtool > Replaced 'Copyright \\d{4}' in src/Admin/Instance/V1/Gapic/InstanceAdminGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/Admin/Database/V1/Gapic/DatabaseAdminGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V1/Gapic/SpannerGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/Admin/Instance/V1/InstanceAdminClientTest.php.
synthtool > No replacements made in src/V1/Gapic/SpannerGapicClient.php for pattern ExecuteSqlRequest_QueryMode, maybe replacement is not longer needed?
synthtool > No replacements made in src/V1/Gapic/SpannerGapicClient.php for pattern ExecuteBatchDmlRequest_Statement, maybe replacement is not longer needed?
synthtool > Replaced 'namespace Google\\\\Cloud\\\\Spanner\\\\Admin\\\\Database\\\\Tests\\\\Unit' in tests/Unit/Admin/Database/V1/DatabaseAdminClientTest.php.
synthtool > Replaced 'namespace Google\\\\Cloud\\\\Spanner\\\\Admin\\\\Instance\\\\Tests\\\\Unit' in tests/Unit/Admin/Instance/V1/InstanceAdminClientTest.php.
synthtool > Replaced '@group database' in tests/Unit/Admin/Database/V1/DatabaseAdminClientTest.php.
synthtool > Replaced '@group instance' in tests/Unit/Admin/Instance/V1/InstanceAdminClientTest.php.
synthtool > Cleaned up 1 temporary directories.
synthtool > Wrote metadata to synth.metadata.

[codecov]

Codecov Report

Merging #2222 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##             master   #2222   +/-   ##
========================================
  Coverage      92.6%   92.6%           
  Complexity     4451    4451           
========================================
  Files           307     307           
  Lines         13271   13271           
========================================
  Hits          12290   12290           
  Misses          981     981

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