feat(cloud): add ruby-cloud-renamed-from for renamed wrapper gems

[suztomo]

Summary

Adds support for the ruby-cloud-renamed-from generator parameter (aliased to :gem.:renamed_from).

Background & Motivation

In most Google Cloud Ruby client libraries, a wrapper gem shares its exact package name with the service (e.g. google-cloud-storage wraps google-cloud-storage-v1).

However, in certain cases a wrapper gem cannot be published under its canonical name due to naming conflicts on RubyGems.org:

1. google-cloud-run: RubyGems already had a pre-existing gem named google_cloud_run. Because RubyGems treats underscores and hyphens as equivalent for gem registration collisions, google-cloud-run could not be registered. The wrapper gem was therefore renamed to google-cloud-run-client.
2. google-iam: RubyGems already had an unrelated gem named google-iam registered in 2011. The wrapper gem was therefore renamed to google-iam-client.

In both cases:

• The wrapper gem name is <canonical-name>-client (e.g. google-cloud-run-client, google-iam-client).
• The wrapped versioned gem(s) use the canonical name (e.g. google-cloud-run-v2, google-iam-v2).
• The Ruby namespaces are canonical (e.g. Google::Cloud::Run, Google::Iam).
• The version constant uses the gem namespace (e.g. Google::Cloud::Run::Client::VERSION, Google::Iam::Client::VERSION in lib/.../client/version.rb).

Previously, this required an .owlbot.rb post-processor script in each gem to rewrite gemspecs, entrypoints, READMEs, and version requires. By adding ruby-cloud-renamed-from, the generator handles this natively for Librarian onboarding.

Generator Behavior with ruby-cloud-renamed-from

When ruby-cloud-renamed-from is specified:

1. gem.namespace: uses Google::Cloud::Run (or Google::Iam) rather than deriving from -client.
2. gem.gem_namespace: uses Google::Cloud::Run::Client (or Google::Iam::Client) for version constants.
3. gem.version_name_full: Google::Cloud::Run::Client::VERSION.
4. gem.versioned_gems: derives dependencies matching <renamed_from>-v* (e.g., google-cloud-run-v2) instead of <name>-v*.
5. gem.google_cloud_short_name: strips google-cloud- from renamed_from (yields "run").
6. Entrypoint template: uses gem.version_name_full in require guards.
7. README template: references versioned gems pattern <renamed_from>-v*.

Resolves googleapis/librarian#7447.

Preview PR in google-cloud-ruby: googleapis/google-cloud-ruby#36528