Support validation of individual map entries when using collectionStrategy=validateMembers

[cduplichien]

We support collection traversal for lists and arrays, so why not maps. Same collectionStrategy semantics would apply. If validating collection members, the validation rule would be passed each Map.Entry as the rule's model. Errors would be registered against the specific map entry.
The map key class will be required to support conversion to/from String using the configured Spring ConversionService. Otherwise, it wouldn't be possible to address the map entries when registering validation errors.