Add option to specify migration status on at the migration site

What does this MR do?

Allow us to specify some Gitaly features as being OPT_OUT.

Some of the Gitaly features are now at the point at which we would like to make them opt-out instead of opt-in.

This change adds a argument at the migration site for us to do this.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?