feat: support created_at filter for the GET /users endpoint (#15633)

Closes https://github.com/coder/coder/issues/12747

We support these filters currently:
https://coder.com/docs/v2/latest/admin/users#user-filtering, adding
`created_at` filter as well.

coderd/users.go

@@ -317,6 +317,8 @@ func (api *API) GetUsers(rw http.ResponseWriter, r *http.Request) ([]database.Us
 		RbacRole:       params.RbacRole,
 		LastSeenBefore: params.LastSeenBefore,
 		LastSeenAfter:  params.LastSeenAfter,
+		CreatedAfter:   params.CreatedAfter,
+		CreatedBefore:  params.CreatedBefore,
 		OffsetOpt:      int32(paginationParams.Offset),
 		LimitOpt:       int32(paginationParams.Limit),
 	})