feat: add new group members endpoint with filtering and pagination (#23067)

Partially addresses #21813 (still need to make changes to the "add user"
button to be complete)

Since there are a lot of user tests already, I moved them into
`coderdtest` to be shared.

enterprise/coderd/coderd.go

@@ -461,6 +461,7 @@ func New(ctx context.Context, options *Options) (_ *API, err error) {
 				)
 
 				r.Get("/", api.groupByOrganization)
+				r.Get("/members", api.groupMembersByOrganization)
 			})
 		})
 		r.Route("/provisionerkeys", func(r chi.Router) {
@@ -545,6 +546,7 @@ func New(ctx context.Context, options *Options) (_ *API, err error) {
 				r.Get("/", api.group)
 				r.Patch("/", api.patchGroup)
 				r.Delete("/", api.deleteGroup)
+				r.Get("/members", api.groupMembers)
 			})
 		})
 		r.Route("/workspace-quota", func(r chi.Router) {