chore(spanner): Add benchwrapper.

[rajatbhatta]

This PR adds a Benchwrapper implementation for Python Spanner Client Library.

The benchwrapper implementation contains the following:

• The proto file, as well as the generated classes from the proto file.
• A file that connects to the emulator using the SPANNER_EMULATOR_HOST environment variable, and implements the RPC’s defined in the proto file.

Checks:

• Appropriate docs were added, wherever necessary.
• Ensure the tests and linter pass.

[zoercai]

Please also get an approval from a Python reviewer before merging, my LGTM is only for what's been implemented, not the Python syntax/readability.

[IlyaFaer]

LGTM

[vi3k6i5]

LGTM

[ansh0l]

This branch is out-of-date with the base branch - please update before merging

[rajatbhatta]

This branch is out-of-date with the base branch - please update before merging

@ansh0l : This is done. Please merge it. Thank you!

[ansh0l]

LGTM.

Minor: Please check if we can name these files (_pb2) better

benchmark/benchwrapper/proto/spanner_pb2.py
benchmark/benchwrapper/proto/spanner_pb2_grpc.py

[rajatbhatta]

LGTM.

Minor: Please check if we can name these files (_pb2) better

benchmark/benchwrapper/proto/spanner_pb2.py benchmark/benchwrapper/proto/spanner_pb2_grpc.py

@ansh0l : These were not written manually but were generated using grpc tool, and the so the files should not be renamed, or changed in any way.

Thank you for merging the PR!