feat: add interfaces for service clients

[bshaffer]

addresses #813

[bshaffer]

Failing "Bazel Build" tests are not caused by these changes

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• tests/Unit/ProtoTests/codegen/samples/BasicClient/a_method.php:25, tag basic_generated_Basic_AMethod_sync
• tests/Unit/ProtoTests/codegen/samples/BasicClient/method_with_args.php:25, tag basic_generated_Basic_MethodWithArgs_sync

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