Add authentication for PostgreSQL wire protocol

[VWagen1989]

We need to add the authentication process under PostgreSQL wire protocol. Otherwise, MyDuck server is vulnerable for malicious attack. And this safty issue would probably be the main concern for the others to have a trial.

[weicao]

Postgres SCRAM auth is already implemented. It is disabled when --superuser-password is empty (the default).

#424 adds a regression test: wrong password is rejected; the correct password can run SELECT 1.