rework auth

lib/auth/credentials.go

lib/auth/credentials/cleartext.go

lib/auth/errors.go

lib/auth/md5/md5.go

lib/auth/md5/md5_test.go

lib/auth/sasl/client.go

lib/auth/sasl/mechanisms.go

lib/auth/sasl/scram/client.go

lib/auth/sasl/scram/mechanisms.go

lib/auth/sasl/scram/server.go

lib/auth/sasl/server.go

lib/bouncer/backends/v0/accept.go

lib/bouncer/frontends/v0/accept.go

lib/gat/configs/pgbouncer/config.go

lib/gat/pooler.go

lib/gat/recipe.go

lib/gat/user.go