Endpoint for project/group access requests

New endpoint for the projects/groups access request API.

This does not verify that the access level granted is a valid access level for projects. Only a subset of the access levels are allowed for projects.

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Groups adds the Minimal and Anonymous levels, but NOT Admin.

I am open to ideas on how to handle this idiomatically, I am still kind of new to Rust and this is at the edge of my knowledge.