`nullable` support for model classes

[hauner]

mapping a json payload to a java class looses the nullable information.

For example the following two payloads will produce the same java object for a model class with a single String property foo:

• {}
• {foo: null}

Both will be deserialized to an object with foo = null. No way to know if it was missing or if it was null.

This is a necessary info to implement PATCH with json merge patch:

• if a property is not set don't change it
• if it is set to "null", clear the property (set it to null).

This could be implemented by using jackson-databind-nullable.

Because of the additional dependency it should be off by default and it gets enabled by setting a new configuration option.