Support target audience

[Schwusch]

When setting a scope with target audience like this

final client = await clientViaServiceAccount(credentials, ['IAP_CLIENT_ID.apps.googleusercontent.com']);

JwtFlow.run() fails with

Exception: Unable to obtain credentials. Invalid response from server.
#0      JwtFlow.run (package:googleapis_auth/src/oauth2_flows/jwt.dart:89:7)
<asynchronous suspension>
#1      clientViaServiceAccount (package:googleapis_auth/auth_io.dart:200:30)

Since the response is structured like this

{
  "id_token": "<JWT_TOKEN>"
}

with missing fields like token_type, expires_in and access_token.

I can find no way of using target audience. Am I missing something?