feat(kms): add ProtectionLevel.External enum; standardize use of 'optional' and 'required' in docstrings (via synth)

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Log from Synthtool

synthtool > Executing /tmpfs/src/git/autosynth/working_repo/kms/synth.py.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:264654a37596a44b0668b8ce6ac41082d713f6ee150b3fc6425fa78cc64e4f20
Status: Image is up to date for googleapis/artman:latest
synthtool > Cloning googleapis.
synthtool > Running generator for google/cloud/kms/artman_cloudkms.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/kms-v1.
synthtool > Copy: /home/kbuilder/.cache/synthtool/googleapis/google/cloud/kms/v1/service.proto to /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/kms-v1/google/cloud/kms_v1/proto/service.proto
synthtool > Copy: /home/kbuilder/.cache/synthtool/googleapis/google/cloud/kms/v1/resources.proto to /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/kms-v1/google/cloud/kms_v1/proto/resources.proto
synthtool > Placed proto files into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/kms-v1/google/cloud/kms_v1/proto.
synthtool > Replaced 'from google.iam.v1 import iam_policy_pb2' in google/cloud/kms_v1/gapic/transports/key_management_service_grpc_transport.py.
.coveragerc
.flake8
MANIFEST.in
docs/_static/custom.css
docs/_templates/layout.html
noxfile.py.j2
setup.cfg
Running session blacken
Creating virtual environment (virtualenv) using python3.6 in .nox/blacken
pip install black==19.3b0
black docs google tests noxfile.py setup.py
reformatted /tmpfs/src/git/autosynth/working_repo/kms/google/__init__.py
reformatted /tmpfs/src/git/autosynth/working_repo/kms/google/cloud/__init__.py
reformatted /tmpfs/src/git/autosynth/working_repo/kms/google/cloud/kms.py
reformatted /tmpfs/src/git/autosynth/working_repo/kms/google/cloud/kms_v1/__init__.py
reformatted /tmpfs/src/git/autosynth/working_repo/kms/google/cloud/kms_v1/gapic/enums.py
reformatted /tmpfs/src/git/autosynth/working_repo/kms/google/cloud/kms_v1/gapic/transports/key_management_service_grpc_transport.py
reformatted /tmpfs/src/git/autosynth/working_repo/kms/google/cloud/kms_v1/proto/resources_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/kms/google/cloud/kms_v1/gapic/key_management_service_client_config.py
reformatted /tmpfs/src/git/autosynth/working_repo/kms/google/cloud/kms_v1/proto/service_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/kms/google/cloud/kms_v1/types.py
reformatted /tmpfs/src/git/autosynth/working_repo/kms/noxfile.py
reformatted /tmpfs/src/git/autosynth/working_repo/kms/google/cloud/kms_v1/gapic/key_management_service_client.py
reformatted /tmpfs/src/git/autosynth/working_repo/kms/tests/unit/gapic/v1/test_key_management_service_client_v1.py
reformatted /tmpfs/src/git/autosynth/working_repo/kms/google/cloud/kms_v1/proto/resources_pb2.py
reformatted /tmpfs/src/git/autosynth/working_repo/kms/google/cloud/kms_v1/proto/service_pb2.py
All done! ✨ 🍰 ✨
15 files reformatted, 6 files left unchanged.
Session blacken was successful.
synthtool > Wrote metadata to synth.metadata.