Add `shared_projects` endpoint

What does this MR do?

This adds a /groups/*group_id/-/shared_projects endpoint, available trough the group_shared_projects_path(group, format: :json) helper.

It responds with projects visible to the current user shared with selected group.

It is possible to pass the sort and filter param, like on the children endpoint, the archived param is not supported, as discussed in https://gitlab.com/gitlab-org/gitlab-ce/issues/44005

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
  • I didn't add a changelog as I think that should be added when the user facing change (the updated frontend) is introduced.
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer

What are the relevant issue numbers?

This is the backend for https://gitlab.com/gitlab-org/gitlab-ce/issues/44005

Closes #46800 (closed)