This pull request addresses a critical gap in the existing functionality of the Google Library in PHP by introducing support for managing schema revisions within Google Pub/Sub. Currently, this feature is missing, and this PR aims to rectify that by providing the ability to retrieve all revisions for a specific schema, as well as the capability to commit and delete these revisions.

Key Changes:

Schema Revision Retrieval: We've added a new method to the Google Library that allows users to retrieve all revisions associated with a particular schema. This enhancement makes it easier to track and manage schema changes over time.

Example Usage:

$client = new PubSubClient();
$schema = $client->schema('schema_name');
$revisions = $schema->listRevisions();

foreach ($revisions['schemas'] as $revision) {
    echo $revision['definition'];
}

Commit and Delete Revisions: To provide comprehensive schema management, we've introduced methods for committing and deleting schema revisions. These functions empower users to control their schema history effectively.

Example Usage (Committing a Revision):

$client = new PubSubClient();
$schema = $client->schema('schema_name');
$definition = file_get_contents('path_to_your_schema_definition.json');
$revision = $schema->commit($definition, $typw);

$client = new PubSubClient();
$schema = $client->schema('schema_name');
$revisions = $schema->deleteRevision('revision_id);

Benefits:

Improved Schema Management: Users can now effortlessly access, commit, and delete schema revisions, enhancing the overall schema management experience.
Comprehensive Control: This addition provides a holistic solution for handling schema revisions within Google Pub/Sub, ensuring better version control and traceability.
We believe that this PR will significantly improve the functionality and usability of the Google Library in PHP, making it a more powerful tool for developers working with Google Pub/Sub schemas. Your feedback and review on this PR are highly appreciated.