feat(firestore): async snippets (GoogleCloudPlatform#4454)

* feat(firestore): create async samples directory

* feat(firestore): update requirements to point to wip git branch

* feat(firestore): copy sync snippets to async client directory

* feat(firestore): create async snippets

* feat(firestore): add _async suffix

* fix: bump copyright year

Co-authored-by: Christopher Wilcox <[email protected]>

Author: rafilong

firestore/cloud-async-client/requirements-test.txt

@@ -0,0 +1,3 @@
+pytest==6.0.1
+pytest-asyncio==0.14.0
+flaky==3.7.0

firestore/cloud-async-client/requirements.txt

@@ -0,0 +1 @@
+git+https://github.com/googleapis/python-firestore.git@v2-staging  # google-cloud-firestore==2.0.0