Using HTTP for AuthUI and Authflow integration

When releasing the new Authentication flow API,
we have made the default Auth UI to employ the new set of API so we could:
1. unify the backend for custom and default AuthUI and reduce future maintenance effort
2. Test the API out

However the integration was done with internal functions, which is not an good representation of how the API would be used.
This hinder ourselves to accomplish the 2nd goal of this exercise.

Therefore it's suggested to change and implement the integration by actual HTTP calls.