Zoekt: Create chatops command for managing replicas

Background

This is an issue to allow us to specify the number of replicas on production without requiring a C2 CR.

Proposal

Now, we should introduce a new chatops command / internal API to allow us changing the number of replicas on production (override). This is to enable this feature for gitlab-org before we assess storage impact and ready roll it out further.

I think that we should add:

/chatops run zoekt enabled_namespace set_replicas <root_namespace_id/group_path> <number_of_replicas/nil>

That would update zoekt_enabled_namespace.replicas override

Context

• Examples:
  • Admin API MR: !116650 (merged)
  • Chatops MR: gitlab-com/chatops!368 (merged)