Provide alternative message code resolver styles

[philwebb]

Introduce new 'style' property to DefaultMessageCodesResolver allowing
for alternative message styles. Current styles are PREFIX_ERROR_CODE
and POSTFIX_ERROR_CODE. The default style retains existing behavior.

Issue: SPR-9707

[philwebb]

@cbeams @jhoeller Any comments on the use of inner static enum (see some but not much precedence in the existing code). I don't like the idea of a boolean flag because it is not easy to extend. Also feedback on the names welcome.

[cbeams]

Thanks Phil, this is merged. Take a look at 38bfb2b to see a couple naming changes, and a combined interface/enum-based approach to preserve both convenience and flexibility (indeed, per your comment above, ensuring that the solution is easily extensible -- not only by us but by users too).