Support id_token for JWT path

I tried to generate an id_token to test Google Cloud Run, but

```
oauth2l fetch --type=jwt --audience="https://hello-my-cloud-run.a.run.app" --credentials=$HOME/Documents/my-service-account-private-key.json --output_format=json
{
"access_token": ".......",
"expiry": "2021-07-28T01:07:05.242408872-07:00",
"token_type": "Bearer"
}

```
It only outputs the access_token, but no id_token.

I tried this token in curl against my cloud run url and got error:

```
< www-authenticate: Bearer error="invalid_token" error_description="The access token could not be verified"
```