Add since and before/until to GET /users/:user_id/contributed_projects

Proposal

A customer has requested that we add attributes to the Projects API: List projects a user has contributed to so that they can filter on the user's specific contribution/activity time frame in the projects returned.

since and before/until would be appropriate attributes to use for them so that they can show only the projects within the user's specific activity time frame. The Commits API has something similar: https://docs.gitlab.com/ee/api/commits.html#list-repository-commits