Draft: Create a new resource for managing pull mirror configurations

Patrick Ricerequested to merge
new-pull-mirror-resource into main

Closes #6566

This MR adds a new resource for configuring pull mirroring via the new Pull Mirror API. It also deprecates the existing gitlab_project_mirror, and adds a new mirrored resource as gitlab_project_push_mirror

Draft

Currently this resource needs a client-go update, because the mirror struct doesn't expose the proper attribute yet.

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