GCP integrator charm doesn't support oauth2 credential type

[Harry Pidcock]

When using the GCP integrator charm with "oauth2" creds we get the following error

unit-gcp-integrator-0: 05:09:32 ERROR unit.gcp-integrator/0.juju-log Hook error:
Traceback (most recent call last):
  File "/var/lib/juju/agents/unit-gcp-integrator-0/.venv/lib/python3.10/site-packages/charms/reactive/__init__.py", line 74, in main
    bus.dispatch(restricted=restricted_mode)
  File "/var/lib/juju/agents/unit-gcp-integrator-0/.venv/lib/python3.10/site-packages/charms/reactive/bus.py", line 390, in dispatch
    _invoke(other_handlers)
  File "/var/lib/juju/agents/unit-gcp-integrator-0/.venv/lib/python3.10/site-packages/charms/reactive/bus.py", line 359, in _invoke
    handler.invoke()
  File "/var/lib/juju/agents/unit-gcp-integrator-0/.venv/lib/python3.10/site-packages/charms/reactive/bus.py", line 181, in invoke
    self._action(*args)
  File "/var/lib/juju/agents/unit-gcp-integrator-0/charm/reactive/gcp.py", line 40, in get_creds
    toggle_flag("charm.gcp.creds.set", layer.gcp.get_credentials())
  File "/var/lib/juju/agents/unit-gcp-integrator-0/charm/lib/charms/layer/gcp.py", line 55, in get_credentials
    creds_data = creds["credential"]["attributes"]["file"]
KeyError: 'file'

$ juju-exec "credential-get"
type: gce
name: google
region: us-east1
endpoint: https://www.googleapis.com
identityendpoint: ""
storageendpoint: ""
credential:
  authtype: oauth2
  attributes:
    client-email: <email address hidden>
    client-id: 123456789-snip.apps.googleusercontent.com
    private-key: |
      -----BEGIN PRIVATE KEY-----
      MIIC...8=
      -----END PRIVATE KEY-----
    project-id: snippy-snip-1234
  redacted: []
cacertificates: []
skiptlsverify: false
iscontrollercloud: true

$ cat .local/share/juju/credentials.yaml
credentials:
  google:
    credentials:
      auth-type: oauth2
      project-id: snippy-snip-1234
      private-key: |
        -----BEGIN PRIVATE KEY-----
        MIIC...8=
        -----END PRIVATE KEY-----
      client-email: <email address hidden>
      client-id: 123456789-snip.apps.googleusercontent.com

Obviously there is no problem with jsonfile creds.