Port GetTagMessages from Gitaly-Ruby to Gitaly

What does this MR do?

This MR provide new feature flag for Gitaly to switch between old implementation of gRPC call GetTagMessages based on Ruby to own Gitaly implementation.
The amount of calls for old and new implementations could be found as Prometheus metric gitaly_get_tag_messages_total with labels ruby and go correspondingly.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.