Add a new API endpoint listing all the contributors of a project
In addition to who they are, we're also providing here their level of
access in detail (ie: including the pattern that applies to the
collaborators).
This endpoint will be useful in the dist-git use-case of pagure
Relates to: https://pagure.io/fedora-infrastructure/issue/9427
Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>