Remove dependency on google-apitools package in Python SDK.

[kennknowles]

AFAIK google-apitools is not officially supported. For Google Cloud Platform APIs such as Datastore, Cloud Storage or Pub/Sub, using Cloud Client Libraries for Python is recommended. If that not possible, we may be able to use https://github.com/google/google-api-python-client - officially supported, but now in maintenance mode.

Imported from Jira BEAM-4850. Original Jira may contain additional context.
Reported by: tvalentyn.

[BjornPrime]

.take-issue

[BjornPrime]

Will be using this as a tracking issue for removing the apitools-generated clients and other stray references to apitools, so that we can eventually remove it as a dependency.

[BjornPrime]

Replace storage_v1_client with GCS client #25676

[BjornPrime]

Replace bigquery_v2_client with Google Cloud BigQuery client (and associated libraries) #25678

[BjornPrime]

Replace dataflow_v1b3_client with a Google Cloud Python client #25679

[BjornPrime]

Replace cloudbuild_v1_client with Google Cloud Build client #25680

[BjornPrime]

Remove remaining dependency on apitools #25681

[Abacn]

Some context: https://s.apache.org/beam-python-dependencies-pm
in particular this action item