Pass Ruby-specific feature flags to the Ruby server

This will pass the value for Ruby-specific flags to the Ruby server via metadata.

Background:

Per gitlab#198270, we want to introduce some additional options and behavior around push mirroring. All of this logic is currently isolated in Gitaly's Ruby codebase, and unlikely to be moved soon. As a result, I wanted to put the new behavior behind feature flags so that we can easily disable it if it's not behaving as expected.

There was no current way to do this. The implementation seen here was suggested by @zj in Slack (ephemeral link, disappears 2020-04-29).

Admittedly this is a little hacky. I'm open to other suggestions.