Videointelligence synth: add support for speech transcription feature

[dpebot]

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:

synthtool > Cloning googleapis.
synthtool > Running generator for google/cloud/videointelligence/artman_videointelligence_v1beta1.yaml.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/video-intelligence-v1beta1.
synthtool > Running generator for google/cloud/videointelligence/artman_videointelligence_v1beta2.yaml.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/video-intelligence-v1beta2.
synthtool > Running generator for google/cloud/videointelligence/artman_videointelligence_v1p1beta1.yaml.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/video-intelligence-v1p1beta1.
synthtool > Running generator for google/cloud/videointelligence/artman_videointelligence_v1p2beta1.yaml.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/video-intelligence-v1p2beta1.
synthtool > Running generator for google/cloud/videointelligence/artman_videointelligence_v1.yaml.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/python/video-intelligence-v1.
synthtool > Replaced "'google-cloud-video-intelligence', \\).version" in google/cloud/videointelligence_v1beta1/gapic/video_intelligence_service_client.py.
synthtool > Replaced "'google-cloud-video-intelligence', \\).version" in google/cloud/videointelligence_v1/gapic/video_intelligence_service_client.py.
synthtool > Replaced "'google-cloud-video-intelligence', \\).version" in google/cloud/videointelligence_v1p1beta1/gapic/video_intelligence_service_client.py.
synthtool > Replaced "'google-cloud-video-intelligence', \\).version" in google/cloud/videointelligence_v1p2beta1/gapic/video_intelligence_service_client.py.
synthtool > Replaced "'google-cloud-video-intelligence', \\).version" in google/cloud/videointelligence_v1beta2/gapic/video_intelligence_service_client.py.
synthtool > Cleaned up 0 temporary directories.