Add git_v2 flag to GitalyClient

Feature flag to be used by Gitaly in order to enable/disable Git v2 protocol.

Part of https://gitlab.com/gitlab-org/gitlab-ce/issues/46555

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides