Support generating multiple files instead of nesting in definitions

[robzienert]

I have a use case where I'm generating schema files from a Java module, of which many of the classes share common property types. Instead of generating a massive schema file for each root class with a list of definitions sub-schemas, I want to be able to generate each type to its own file and reference these types.

For this to work, I'd expect that the generator could either be provided a $id for each type via an annotation (or infer it based on the class name), then use this value in $ref. The generator config itself could be given a root id path as well to prepend to the ids for each class type.

Thinking something like so:

@JsonSchemaId("person")
class Person {
  String name;
}

@JsonSchemaId("car")
class Car {
  Person driver;
  Person passenger;
}

Would then generate two files, one for Person and another for Car.

This behavior would then be enabled (to support backwards compatibility), via the config (with a better name):

var config = JsonSchemaConfig.nullableJsonSchemaDraft4();
config.createMultipleFiles(true);

Given that generateJsonSchema produces a JsonNode, with this particular flag enabled it would not include definitions, and would expect that other schema files that it needs for a fully valid schema would be generated separately (with subsequent calls to the same method, but with a different Class or JavaType).