support json schema generation for @JsonUnwrapped (#271)

[ciderale]

This pull request relates to #271 which only fixed the "old" schema generation method.
The "new" json schema generation approach (jackson-module-jsonSchema) did not yet handle the JsonUnwrapped annotation.
This patch adds this support by overriding the depositSchemaProperty method of UnwrappingBeanPropertyWriter.
An updated test for verifying the correct behavior is in a separate commit (in jackson-module-jsonSchema).

[cowtowncoder]

Sounds good. As per my comments on FasterXML/jackson-module-jsonSchema#61 all we need is the CLA and I can merge this in 2.4, as well as push it forward to 2.5 and master.

[cowtowncoder]

Received CLA, will proceed with merge.

[cowtowncoder]

I think I'll want to change JsonFormatVisitorNullWrapper a bit, however. It probably belongs to different package. And name is probably bit misleading as it has nothing to do with nulls.
But it is not used by code in the patch so it is bit hard to say.

[cowtowncoder]

I'll rename it as JsonFormatVisitorWrapper.Base, to follow pattern used by other similar things in Jackson core.

[ciderale]

I agree that this rename makes sense. I thought about that as well, but decided to keep my change minimal (and not exposing anything new with public visibility).

[cowtowncoder]

Yeah I can see both sides; since it's for now only needed for specific use... anyhow, thank you for fixing this. I'll merge other patch next.