feat(firestore): add v1beta1 deprecation annotation (#34)

* Added v1beta1 deprecation annotation

* firestore: add v1beta1 deprecation annotation

Co-authored-by: Christopher Wilcox <crwilcox@google.com>

Author: mf2199

google/cloud/firestore_v1beta1/__init__.py

@@ -15,6 +15,7 @@
 """Python idiomatic client for Google Cloud Firestore."""
 
 from pkg_resources import get_distribution
+import warnings
 
 __version__ = get_distribution("google-cloud-firestore").version
 
@@ -40,6 +41,13 @@
 from google.cloud.firestore_v1beta1.watch import Watch
 
 
+_V1BETA1_DEPRECATED_MESSAGE = (
+    "The 'v1beta1' API endpoint is deprecated. "
+    "The client/library which supports it will be removed in a future release."
+)
+warnings.warn(_V1BETA1_DEPRECATED_MESSAGE, DeprecationWarning)
+
+
 __all__ = [
     "__version__",
     "ArrayRemove",