elasticsearch: Add support for an experimental repository indexer

What does this MR do?

Performs elasticsearch repository indexing using gitlab-elasticsearch-indexer rather than bin/elastic_repo_indexer if enabled

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

Need to check with the build team that the path is acceptable.

Why was this MR needed?

For 9.1, we're hoping to switch from a Ruby-based indexer to a Go-based one. If it causes problems, it would be useful to be able to switch back to the Ruby one immediately.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• [ ] API support added feature flag, to be removed soon
• 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?

Closes #1606 (closed)