[Resource group API] Get all resource groups in a project

Problem

Users have no idea how many resource groups exist in a project. This causes a usability issue, like https://gitlab.slack.com/archives/CBZGR90A2/p1650997207284899?thread_ts=1650990734.035509&cid=CBZGR90A2 (internal).

Proposal

Add GET /projects/:id/resource_groups endpoint to return all resource groups in a project. Pagination should be supported.