* feat(firestore): Expose Async Firestore Client. (#621)

* feat(firestore): Expose Async Firestore Client.

* fix: Added type hints and defintion wording changes

* fix: removed future annotations until Python 3.6 is depreciated.

* fix: added missed type and clarifying comment for Python 3.6 type hinting.

* fix: lint

* Adds integration tests for the Async Firstore module (#623)

* Add integration tests for async firstore module

* fix: made pytest Python 3.6 compatible

* Trigger Integration Tests

* fix: correct copyright year

* Add code snippets for firestore modules. (#628)

* Add code snippets for firestore modules.

* fix: clarified snippet names and fixed newline.

* fix: Removed var tags. These won't work as I intended it to since html is escaped when using includecode.

Co-authored-by: Lahiru Maramba <llahiru@gmail.com>