Add the SASL PLAIN verification exchange

fedserv now handles the SASL exchange an ircd relays over ENCAP (modes H/S/C/D):
on PLAIN it answers the empty challenge, decodes the client payload, and verifies
the credentials against the account store, replying D S or D F. Wiring the
account login on success and advertising the mechanism list to the ircd are the
remaining integration steps.

Cargo.toml

@@ -10,6 +10,7 @@ serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 toml = "0.8"
 argon2 = { version = "0.5", features = ["std"] }
+base64 = "0.22"
 anyhow = "1"
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }