Allow specifying the order of operations in AffineTransform #8601

At present, the AffineTransform class in astropy.coordinates assumes that the order of transformations is "rotation then offset," but we should probably allow this to be set. Alternatively, we could support rotations on either end of the offset (i.e., rotate-offset-rotate) and have the rotations default to np.eye(). This would help speed up some of the FunctionTransformWithFiniteDifference transformations in the ecliptic frames (see #8394).