GCP CLI support: add support for gsutil.

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This PR adds changes necessary to support gsutil v2 in GCP App Access. gsutil v2 does not support CLOUDSDK_AUTH_ACCESS_TOKEN; instead, we have to write the token to file and set env variables so that gsutil reads the configuration file, which points at the file with token.

gsutil v3 is based on GCP Cloud SDK and, therefore, already supported.

More info: GoogleCloudPlatform/gsutil#357 (comment)

This change is a follow-up to #17257.

Setup: see #19790.

Usage options:

1. Directly call gsutil using tsh as wrapper:

> tsh gsutil ls
gs://example-gcp-bucket-001/
gs://example-gcp-bucket-002/

2. Start proxy in one console. Use gsutil in another console after setting env variables:

> tsh proxy gcp --port 3500

Started GCP proxy on http://127.0.0.1:3500.

Use the following credentials and HTTPS proxy setting to connect to the proxy:

  export BOTO_CONFIG=/Users/username/.tsh/gcp/teleport.example.com/gcp/5b46ada1_boto.cfg
  export CLOUDSDK_AUTH_ACCESS_TOKEN=8db77684aea05ef3c49ca0a313d5d9fd
  export CLOUDSDK_CORE_CUSTOM_CA_CERTS_FILE=/Users/username/.tsh/keys/teleport.example.com/username-app/teleport.example.com/gcp-localca.pem
  export HTTPS_PROXY=http://127.0.0.1:3500

> export BOTO_CONFIG=/Users/username/.tsh/gcp/teleport.example.com/gcp/5b46ada1_boto.cfg
> export CLOUDSDK_AUTH_ACCESS_TOKEN=8db77684aea05ef3c49ca0a313d5d9fd
> export CLOUDSDK_CORE_CUSTOM_CA_CERTS_FILE=/Users/username/.tsh/keys/teleport.example.com/username-app/teleport.example.com/gcp-localca.pem
> export HTTPS_PROXY=http://127.0.0.1:3500
> gsutil ls
gs://example-gcp-bucket-001/
gs://example-gcp-bucket-002/