addresses #21808 by adding a workflow to sync the PHP extension to a subrepo. After this PR is merged, we will need to:

• create the repo protocolbuffers/protobuf-php-ext,
• submit that repo to packagist.org after it is tagged.

Users will then be able to install the protobuf extension using pie instead of pecl:

pie install google/protobuf-ext

New

• php/ext/google/protobuf/README.md: Readme for the mirrored google/protobuf-ext PIE extension package explaining installation (pie install google/protobuf-ext) and redirecting issues/PRs to the main repository.
• php/ext/google/protobuf/composer.json: Package metadata for the PIE PHP extension
• php/ext/google/protobuf/package_mirror.py: Python script that copies and structures C extension source files (*.c, *.h, *.inc, *.m4, *.w32, composer.json, README.md, LICENSE) along with required third_party/utf8_range dependencies into a target mirror directory.
• php/ext/google/protobuf/test_pie_package.sh: Test script that downloads pie.phar, populates a temporary local repository using package_mirror.py, adds it via pie repository:add path, and executes pie build google/protobuf-ext:@dev.

Modified

• .github/workflows/update_php_repo.yml: Added the update-php-ext-repo job to automatically sync C extension files to protocolbuffers/protobuf-php-ext

See grpc/grpc#42784 for similar work done for the gRPC extension