feat(internal/librarian/php): add inital PHP client library generator

[zhumin8]

An initial implementation of the PHP client library generator is added to librarian. The goal of this change is to enable executing the PHP GAPIC generator as a protoc plugin from librarian's generation path.

The generator setup downloads the PHP generator codebase, installs its Composer dependencies, and generates a wrapper script that delegates compilation to the Main.php entrypoint. (Will remove this setup in followups)
Compilation parameters such as transport settings, numeric enums, and migration modes are automatically derived and passed during execution.

For #6629

[gemini-code-assist]

Code Review

This pull request implements the PHP client library generator, which fetches the PHP generator, installs Composer dependencies, writes a wrapper script, and runs the generator plugin via protoc. It also adds integration tests to verify the generation output. The review feedback suggests avoiding race conditions and Windows path issues by checking if the wrapper script exists before writing it and using filepath.ToSlash. Additionally, it recommends using errors.Is instead of os.IsNotExist and properly propagating other errors when checking the vendor directory.

[zhumin8]

There seems to be an issue with our yaml formatter removing empty lines, and php generator having issue without the blank lines in service yaml? Investigating.

[zhumin8]

Adjusted test coverage temporarily, will do followups to refactor and add tests. In the meantime, installGenerator is intended to be throw away code while proper install command is ready (not adding test for it).