Add username and password handling from user mapping

[Hunaid2000]

User/password options are in server options which is not according to the standard. We retrieve username and password from user mapping instead of server options.

Those using this FDW need to enable auth, setup roles/users (like root) on etcd, as mentioned in docs. Then create a user mapping for that user with password like

CREATE USER MAPPING FOR CURRENT_USER SERVER my_etcd_server OPTIONS (user 'root', password 'secret');