chore: Make Union meta overridable (#1583)

This PR makes the Union Options configurable, similar to how it works with ObjectTypes
---------

Co-authored-by: Erik Wrede <erikwrede@users.noreply.github.com>

Author: that-one-arab

graphene/types/union.py

@@ -51,12 +51,14 @@ class Query(ObjectType):
     """
 
     @classmethod
-    def __init_subclass_with_meta__(cls, types=None, **options):
+    def __init_subclass_with_meta__(cls, types=None, _meta=None, **options):
         assert (
             isinstance(types, (list, tuple)) and len(types) > 0
         ), f"Must provide types for Union {cls.__name__}."
 
-        _meta = UnionOptions(cls)
+        if not _meta:
+            _meta = UnionOptions(cls)
+
         _meta.types = types
         super(Union, cls).__init_subclass_with_meta__(_meta=_meta, **options)