Call hook rpcs from operations service

fixes: #2637 (closed)

in !1740 (merged), we enabled the gitaly-hooks binary to call the Hook RPCs through an internal gitaly socket if a feature flag is set.

Another code path through Gitaly that runs the hooks are the operations service RPCs. However, these take a different path to the hooks as it goes through the ruby hook.rb. We need to pass a feature flag all the way through so that hook.rb will call the gitaly-hooks binary with the correct environment variables so to allow gitaly-hooks to call the hook service RPCs.

we already have the ability to read ruby specific feature flags on the gitaly ruby side. This MR allows gitaly-feature-ruby-call-hook-rpc feature flag, when set, to trigger hook.rb to set the correct environment variables for gitaly-hooks.