Any support (#50)

* Make MessagePool easily importable

* Fix import error

* Define global message pool

* Update generated code

* Add test

* Move test imports

* Don't use a global message pool in the lib

* Add missing file

Author: AdrienVannson

src/betterproto2/__init__.py

@@ -1,6 +1,6 @@
 from __future__ import annotations
 
-__all__ = ["__version__", "check_compiler_version", "unwrap"]
+__all__ = ["__version__", "check_compiler_version", "unwrap", "MessagePool"]
 
 import dataclasses
 import enum as builtin_enum
@@ -43,6 +43,7 @@
 from dateutil.parser import isoparse
 from typing_extensions import Self
 
+from betterproto2.message_pool import MessagePool
 from betterproto2.utils import unwrap
 
 from ._types import T