Support multiple audience client IDs in idToken validator

[anton-kstnk]

I was going through the idToken validation docs here: https://developers.google.com/identity/sign-in/web/backend-auth and all the code examples suggest that there is a possibility of multiple clientIDs (aka audience) validation, i.e. in Java:

GoogleIdTokenVerifier verifier = new GoogleIdTokenVerifier.Builder(transport, jsonFactory) // Specify the CLIENT_ID of the app that accesses the backend: .setAudience(Collections.singletonList(CLIENT_ID)) // Or, if multiple clients access the backend: //.setAudience(Arrays.asList(CLIENT_ID_1, CLIENT_ID_2, CLIENT_ID_3)) .build();

This seems not to be supported by the Golaang client:
func (v *Validator) validate(ctx context.Context, idToken string, audience string) (*Payload, error)

if audience != "" && payload.Audience != audience { return nil, fmt.Errorf("idtoken: audience provided does not match aud claim in the JWT" }

I see 2 possible options of such implementation:

1. Create a new validate function that handles an array of strings.
2. Inside of the existing function, check if we can split audience string by comma delimiter and create an array which will be processed after, assuming that user can pass a string of comma-separated audiences.

[quartzmo]

@anton-kstnk Thank you for pointing out this issue in the idtoken package. I agree that it appears the API should probably be expanded to accommodate multiple audience values. My preference would be for a variadic or array/slice input. I think we should avoid splitting the string.

In the meantime, the page you linked shows a simple workaround for the Python library that I believe would also serve for Go:

    # Specify the CLIENT_ID of the app that accesses the backend:
    idinfo = id_token.verify_oauth2_token(token, requests.Request(), CLIENT_ID)

    # Or, if multiple clients access the backend server:
    # idinfo = id_token.verify_oauth2_token(token, requests.Request())
    # if idinfo['aud'] not in [CLIENT_ID_1, CLIENT_ID_2, CLIENT_ID_3]:
    #     raise ValueError('Could not verify audience.')