Include mypy stubs for protos in cirq packages. (#2383)
Include mypy stubs for protos in cirq packages. (#2383)

This allows typechecking of protos in code that depends on cirq now that we ship a `py.typed` file.
quantumlib/Cirq,balopat/Cirq,balopat/Cirq,quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,quantumlib/Cirq,balopat/Cirq