GitLab should pass through feature flags to gitlab-elasticsearch-indexer to pass through to Gitaly

From this discussion in Slack.

We need GitLab rails to pass some Gitaly specific feature flags down to gitlab-elasticsearch-indexer so that it can pass those through to Gitaly via gRPC calls. We already send the correlation id so it can follow the same approach to pass it through.

Technical details

1. Docs for gitaly feature flags https://gitlab.com/gitlab-org/gitaly/-/blob/master/doc/PROCESS.md#feature-flags-in-gitaly
2. GitLab passes data to gitlab-elasticsearch-indexer via env vars https://gitlab.com/gitlab-org/gitlab/-/blob/2c240fd3e0e0648733b08b6dec6acaae75aeec63/ee/lib/gitlab/elastic/indexer.rb#L155
3. gitlab-elasticsearch-indexer constructs a gitaly client for gRPC calls https://gitlab.com/gitlab-org/gitlab-elasticsearch-indexer/-/blob/c33cd08b6239fb9e3bc86302f27037d00c647049/git/gitaly.go#L96