[C-API] Google credentials does not support. both IdToken and AccessCode

[cmelchior]

Currently, the C-API only exposes:

RLM_API realm_app_credentials_t* realm_app_credentials_new_google(const char* id_token) RLM_API_NOEXCEPT;

But Google and ObjectStore support two different tokens:

AppCredentials::google(AuthCode(StringData(id_token).data()));
AppCredentials::google(IdToken(StringData(id_token).data()));

So we need an API that can distinguish between these. Maybe something like

RLM_API realm_app_credentials_t* realm_app_credentials_new_google_id_token(const char* id_token) RLM_API_NOEXCEPT;
RLM_API realm_app_credentials_t* realm_app_credentials_new_google_auth_code(const char* auth_code) RLM_API_NOEXCEPT;