Action is hardcoded in calls to `/internal/allowed`

When checking whether a change is allowed to go through we use Rails' /internal/allowed API endpoint. In order to give Rails the ability to judge based on different parameters we send along some information in that request:

• The protocol determines whether it was a "web" operation or an "http"/"ssh"-based push.
• Changes contains the list of references that have changed.
• The project's path so that Rails knows which project this is.
• An "action" to tell Rails what has happened.

The "action" is hardcoded to git-receive-pack though, which makes it a rather useless signal.

We should fix the action to become more meaningful and tell the client what exactly has happened. The proposal is described in #4581 (comment 2116525724).

The steps would be the following:

1. Decide on which metadata key to recognize for Gitaly clients to pass in metadata through the gRPC call

2. https://gitlab.com/gitlab-org/gitaly/-/blob/master/internal/gitlab/http_client.go#L167 is what contains the call to set "action". Replace this with the value from the gRPC metadata.

   • NOTE: When Git hooks are invoked by Git itself, it encodes a payload eg: https://gitlab.com/gitlab-org/gitaly/-/blob/master/internal/git/gitcmd/hooks_options.go#L108 so the context will need to be serialized/de-serialized

3. Document this in the RPC docs that end up calling PreReceiveHook.