Include mypy types

[devnev]

Is your feature request related to a problem? Please describe.

Modern python can be type-checked using mypy, but the python distribution of this package does not include the type specs, nor is there a companion type package.

Describe the solution you'd like

Types generated using e.g. https://pypi.org/project/mypy-protobuf/ included either directly in the python googleapis-common-protos package or a separate googleapis-common-protos-stubs package. A separate stubs package might help with avoiding accidental breakage.

Describe alternatives you've considered

I could check in the googleapi protos into the repository where I'm using them and add codegen from that.

Additional context
For the grpcio packages, there's stubs maintained by a third-party, https://pypi.org/project/grpc-stubs/. The protoc plugin https://pypi.org/project/mypy-protobuf/ codegens stubs for proto files (messages and grpc clients/handlers).