Fixed #22001 -- Ensure db_type is respected.

db_parameters should respect an already existing db_type method and
return that as its type string. In particular, this was causing some
fields from gis to not be generated.

Thanks to @bigsassy and @blueyed for their work on the patch.

Also fixed #22260

tests/field_subclassing/fields.py

@@ -74,3 +74,8 @@ class JSONField(six.with_metaclass(models.SubfieldBase, models.TextField)):
         if value is None:
             return None
         return json.dumps(value)
+
+
+class CustomTypedField(models.TextField):
+    def db_type(self, connection):
+        return 'custom_field'