[Task] Move mock-related C code to a test extension

[jduo]

Description

• Build a separate extension for test code
• Move the call to register_mock_constructor_class(); from valkey_glide.c to the new test extension's initializer function
• Add a test building configuration to the Makefile
• Move compilation of client_constructor_mock.c and generation from client_constructor_mock.stub.php to the test build configuration
• (Nice to have) Generate protobuf classes for PHP in the test building configuration in the Makefile.
• Add the test extension to the php.ini
• Update documentation for any changes in steps.

Checklist

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Additional Notes

No response

[github-actions]

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