…1551)

…le (#1552)

Enable self signed jwt if google-auth service account credentials are used.

Tested with Storage, Compute and PubSub APIs:

```
import googleapiclient.discovery

project = "<project>"
zone = "us-west1-a"

compute = googleapiclient.discovery.build('compute', 'v1')
result = compute.instances().list(project=project, zone=zone).execute()
print(result)

storage = googleapiclient.discovery.build('storage', 'v1')
result = storage.buckets().list(project=project).execute()
print(result)

topic = "<topic>"
pubsub = googleapiclient.discovery.build('pubsub', 'v1')
result = pubsub.projects().topics().get(topic=f"projects/{project}/topics/{topic}").execute()
print(result)
```

)

Co-authored-by: Anthonios Partheniou <partheniou@google.com>

Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply.github.com>