gitlab: Pass RPC context to internal API calls

Mustafa Bayarrequested to merge
4581-pass-context-from-rpc-to-allowed into master

Rails needs to pass some information via Gitaly to internal APIs such as /internal/allowed and /post_receive. Instead of hardcoding such values we decided to pass metadata from clients that Gitaly will directly relay to internal API. This allows for more flexible and detailed information exchange, and can be easily extended without needing to upgrade Gitaly each time.

Closes #4581 (closed)

Edited by Mustafa Bayar

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