IAM: Add 'IAMCredentialsClient.generate_identity_binding_access_token' via synth.

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:

synthtool > Executing /tmpfs/src/git/autosynth/working_repo/iam/synth.py.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:bfb92654b4a77368471f70e2808eaf4e60f263b9559f27bb3284097322787bf1
Status: Image is up to date for googleapis/artman:latest
synthtool > Cloning googleapis.
synthtool > Running generator for google/iam/credentials/artman_iamcredentials_v1.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/iamcredentials-v1.
synthtool > Replaced 'google-cloud-iamcredentials' in google/cloud/iam_credentials_v1/gapic/iam_credentials_client.py.
synthtool > Replaced 'google-cloud-iamcredentials' in docs/conf.py.
synthtool > Replaced 'from google\\.iam\\.credentials\\.v1 import common_pb2' in tests/unit/gapic/v1/test_iam_credentials_client_v1.py.
synthtool > Replaced 'from google\\.iam\\.credentials\\.v1 import common_pb2' in google/cloud/iam_credentials_v1/types.py.
synthtool > Replaced 'from google\\.iam\\.credentials\\.v1 import common_pb2' in google/cloud/iam_credentials_v1/proto/iamcredentials_pb2_grpc.py.
synthtool > Replaced 'from google\\.iam\\.credentials\\.v1 import common_pb2' in google/cloud/iam_credentials_v1/proto/iamcredentials_pb2.py.
synthtool > Replaced 'from google\\.iam\\.credentials\\.v1 import common_pb2' in google/cloud/iam_credentials_v1/gapic/iam_credentials_client.py.
synthtool > Replaced 'from google\\.iam\\.credentials\\.v1 import iamcredentials_pb2_grpc' in google/cloud/iam_credentials_v1/gapic/iam_credentials_client.py.
synthtool > Replaced 'from google\\.iam\\.credentials\\.v1 import iamcredentials_pb2_grpc' in google/cloud/iam_credentials_v1/gapic/transports/iam_credentials_grpc_transport.py.
synthtool > Replaced '"""Attributes:\n' in google/cloud/iam_credentials_v1/proto/common_pb2.py.
.coveragerc
.flake8
MANIFEST.in
noxfile.py.j2
setup.cfg
Running session blacken
Creating virtualenv using python3.6 in /tmpfs/src/git/autosynth/working_repo/iam/.nox/blacken
pip install --upgrade black
black google tests docs
reformatted /tmpfs/src/git/autosynth/working_repo/iam/google/cloud/__init__.py
reformatted /tmpfs/src/git/autosynth/working_repo/iam/google/__init__.py
reformatted /tmpfs/src/git/autosynth/working_repo/iam/google/cloud/iam_credentials.py
reformatted /tmpfs/src/git/autosynth/working_repo/iam/google/cloud/iam_credentials_v1/__init__.py
reformatted /tmpfs/src/git/autosynth/working_repo/iam/google/cloud/iam_credentials_v1/gapic/iam_credentials_client_config.py
reformatted /tmpfs/src/git/autosynth/working_repo/iam/docs/conf.py
reformatted /tmpfs/src/git/autosynth/working_repo/iam/google/cloud/iam_credentials_v1/proto/common_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/iam/google/cloud/iam_credentials_v1/gapic/transports/iam_credentials_grpc_transport.py
reformatted /tmpfs/src/git/autosynth/working_repo/iam/google/cloud/iam_credentials_v1/proto/iamcredentials_pb2.py
reformatted /tmpfs/src/git/autosynth/working_repo/iam/google/cloud/iam_credentials_v1/types.py
reformatted /tmpfs/src/git/autosynth/working_repo/iam/google/cloud/iam_credentials_v1/proto/iamcredentials_pb2_grpc.py
reformatted /tmpfs/src/git/autosynth/working_repo/iam/google/cloud/iam_credentials_v1/gapic/iam_credentials_client.py
reformatted /tmpfs/src/git/autosynth/working_repo/iam/tests/unit/gapic/v1/test_iam_credentials_client_v1.py
reformatted /tmpfs/src/git/autosynth/working_repo/iam/google/cloud/iam_credentials_v1/proto/common_pb2.py
All done! ✨ 🍰 ✨
14 files reformatted, 3 files left unchanged.
Session blacken was successful.
synthtool > Cleaned up 2 temporary directories.
synthtool > Wrote metadata to synth.metadata.