Add a general-purpose `Contextualizable` interface for all handlers to support

(or, if need be, more than one)

Currently a small set of handlers can be "contextualized": that is, called with context within Object Graph, as specified by `Property` for which handler is related. This set mostly consists of serializers and deserializers.

But there are many other handlers, especially ones added via annotations, that do not get a chance to change their behavior based on context. It seems quite doable to allow use of an add-on interface like, say, `Contextualizable` that would allow (re)configuration in this way.

One interface may or may not be doable, due to:

* Difference between reading (deserialize) and writing (serialize) taking different context object (`DeserializationContext` vs `SerializerProvider`) -- although, they are both subtypes of `DatabindContext` so maybe this is not a fundamental problem.
* Are there other kinds of context beyond property-bound one? Class annotations might just take `JavaType`?