Add an extension point for FormatSchema auto-assignment based on type

[cowtowncoder]

Since many dataformat backends require specifying of a FormatSchema -- at least Avro, CSV, Protobuf and (Java) Properties do -- and since in most cases there is a way to either generate a schema from type definitions, or perhaps fetch it from an external repository, it would make since to allow plugging of such resolvers. It may also make sense to offer default implementations for simple cases, like POJO-based generation.

Such functionality would be especially useful for use cases where ObjectMapper can be configured and passed as configuration: for example to use with JAX-RS message body reader/writer provider. While ability to pass ObjectReader/ObjectWriter instead would work around the issue, sometimes passing factory (that ObjectMapper basically is) works better.

It is not clear if this could fit in 2.9 yet; I'll tag this for 3.x for now.

[guidomedina]

Fixes FasterXML/jackson-dataformat-protobuf#15