feat: Add typing to proto.Message based class attributes

[PicardParis]

Fixes #1379

[PicardParis]

Here is the rationale for the latest changes:

1. proto.RepeatedField
   • The runtime type is proto.marshal.collections.repeated.RepeatedComposite
   • RepeatedComposite is a subclass of MutableSequence:
     • class Repeated(collections.abc.MutableSequence): ...
     • class RepeatedComposite(Repeated): ...
   • See implementation
2. proto.MapField
   • The runtime type is proto.marshal.collections.maps.MapComposite
   • MapComposite is a subclass of MutableMapping:
     • class MapComposite(collections.abc.MutableMapping): ...
   • See implementation
3. Parts of the code do use these attributes as mutable:
   • Example:

     gapic-generator-python/gapic/templates/%namespace/%name_%version/%sub/services/%service/async_client.py.j2

     Line 305 in 1a94843

     request.{{ key }}.extend({{ field.name }})

4. Doc annotations incorrectly indicated Sequence & Mapping
   • See implementation
5. Goal of the changes: Sequence → MutableSequence & Mapping → MutableMapping

[parthea]

I'm going to close and re-open this PR to re-trigger gh actions.

[PicardParis]

...fixing conflicts due to updates in the main branch...

[parthea]

I generated the code locally for datastore and the mypy test failed with these changes.
See generated code here: https://github.com/googleapis/python-datastore/compare/test-local-gapic?expand=1

[PicardParis]

• I made another pass. Tests are successful.
• I was not able to reproduce your issue when generating datastore locally.
  • We must be using different generation commands.
  • I tried

    protoc google/datastore/v1/*.proto \
      --proto_path=../api-common-protos/ --proto_path=. \
      --python_gapic_out=../dest/
    

    and

    protoc google/datastore/admin/v1/*.proto \
      --proto_path=../api-common-protos/ --proto_path=. \
      --python_gapic_out=../dest/
    

    and nox -s mypy completes for both.
  • Which command do you use to generate datastore?
• Anyway, the new pass should be exhaustive. Can you recheck?

[parthea]

LGTM