Support for generating ID token with service account credentials

[h-michael]

Thank you for your great work! I implemented generating ID token function with service account credentials.

Implement IdTokenSourceConfig::new

IdTokenSourceConfig's all fields are private. So There is no way to build IdTokenSourceConfig.

Implement InternalIdToken to deserialize ID token response

ID token endpoint's JSON response is the following schema.

{
  "id_token": "MY_ID_TOKEN"
}

This doesn't include access_token, token_type and expires_in. Therefore, if I try to deserialize it as InternalToken, I get an error.

I know google-cloud-rust follows Google Cloud Go. So I looked into how it is implemented in Google Cloud Go. I found a function below. https://github.com/golang/oauth2/blob/950ef44c6e079baf075030377d90bf0c7e4b7b7a/jwt/jwt.go#L101-L185 If the expected key is not in the JSON payload, Golang sets zero value to the key so that no error occurs when unmarshalling.

To avoid this with Rust we need to make InternalToken.access_token and InternalToken.token_type to Option<String> or need to implement another struct for deserializing ID token response. The former has a little too much influence, so I implement a new struct InternalIdToken for now.

Which implementation method would you prefer or do you have another idea?

[yoshidan]

The former has a little too much influence, so I implement a new struct InternalIdToken for now.

I agree with the current implementation, as it is easier to understand if different fields are received in the response if they are explicitly separated.