Begin including schema dialect specification in produced jsonschema

In jsonschema's documentation they state
> It's not always easy to tell which draft a JSON Schema is using.
> You can use the $schema keyword to declare which version of the JSON Schema specification the schema is written to.
> It's generally good practice to include it, though it is not required.

and

> For brevity, the $schema keyword isn't included in most of the examples in this book, but it should always be used in the real world.

Basically, to know how to parse a schema, it's important to include what
schema dialect is being used for the schema specification. The change in
this commit ensures we include that information.

core/dbt/contracts/util.py

@@ -198,7 +198,7 @@ class VersionedSchema(dbtClassMixin):
     @classmethod
     @functools.lru_cache
     def json_schema(cls) -> Dict[str, Any]:
-        json_schema_obj = build_json_schema(cls, dialect=DRAFT_2020_12)
+        json_schema_obj = build_json_schema(cls, dialect=DRAFT_2020_12, with_dialect_uri=True)
         json_schema = json_schema_obj.to_dict()
         json_schema["$id"] = str(cls.dbt_schema_version)
         return json_schema