Support arbitrary values for extension values

Proto itself supports primitives, not just messages.
See nipunn1313/mypy-protobuf#244 for an example
motivating this change.

Test Plan: I was able to use MYPYPATH to test an updated version of
mypy-protobuf with this change.

stubs/protobuf/google/protobuf/internal/extension_dict.pyi

@@ -4,7 +4,7 @@ from google.protobuf.descriptor import FieldDescriptor
 from google.protobuf.message import Message
 
 _ContainerMessageT = TypeVar("_ContainerMessageT", bound=Message)
-_ExtenderMessageT = TypeVar("_ExtenderMessageT", bound=Message)
+_ExtenderMessageT = TypeVar("_ExtenderMessageT")
 
 class _ExtensionFieldDescriptor(FieldDescriptor, Generic[_ContainerMessageT, _ExtenderMessageT]): ...