Add support to reference external HandlerMethodArgumentResolver beans which might contain already configured instances (e.g. through a 3rd-party namespace handler).

 Users can not mix and match between "inner bean" argument resolver and "external bean" argument resolver. This commit only focuses only on argument-resolver, while the support could be extended to return value handlers as well.

 Issue: SPR-11927
Fix white space issue
Support <ref> for arg resolvers return value handlers

Issue: SPR-11927