[VIO-2757] Implement loopback OAUTH flow

[AdamIsrael]

The OOB OAUTH flow we were using was discontinued by Google in October 2022 oob-migration.

This PR implements the loopback flow recommended for Desktop clients oob-migration/desktop-client, fixing the current sctl breakage.

This doesn't make the headless workflow any easier; that might require a service-account and the server-to-server flow.

This also implements a --port flag to use in case the default port (currently 9999) is in use or otherwise unavailable. This port needs to match the redirect_uris in the configured credentials.

# Note that `redirect_uris` should point to localhost and an available port
$ sctl credential add
{
    "installed": {
        "client_id": "<redacted>",
        "project_id": "myproject",
        "auth_uri": "https://accounts.google.com/o/oauth2/auth",
        "token_uri": "https://oauth2.googleapis.com/token",
        "auth_provider_x509_cert_url": "https://www.googleapis.com/oauth2/v1/certs",
        "client_secret": "<redacted>",
        "redirect_uris": [
            "http://localhost:9999/auth/google/callback"
        ]
    }
}

Go to the following link in your browser then type the authorization code: 
https://accounts.google.com/o/oauth2/auth?access_type=offline&client_id=<redacted>&redirect_uri=http%3A%2F%2F127.0.0.1%3A9999&response_type=code&scope=https%3A%2F%2Fwww.googleap
is.com%2Fauth%2Fcloudkms&state=2lyAnj9cAojVooC5UxnlTg%3D%3D