Allow use of `@JsonAnySetter` on a Map-valued field, no need for setter

[cowtowncoder]

It would seem useful to allow use of @JsonAnySetter on fields with proper value; Map at least should be supported; optionally also ObjectNode but that's just icing on cake.

[cowtowncoder]

Hmmh. Just realized that the implementation has a flaw in that access to @JsonAnySetter does not go via AnnotationIntrospector, like all Annotation access ought to. Will fix for 2.9, related to also making it possible to disable any-setter mutator.