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

[cowtowncoder]

(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?

[cowtowncoder]

Handlers to consider:

• Converters
• Custom filters (via @JsonInclude)
• ValueInstantiators