proxy: introduce "quota" error classification

[conradludgate]

To distinguish between errors

• user - a user made a mistake
• ratelimit - a user issued too many requests
• quota - a user used more than they are paying for

https://github.com/neondatabase/neon/blob/1afab13ccb95ed083397c5bff1e31ae1631b1091/proxy/src/error.rs#L36 https://github.com/neondatabase/neon/blob/1afab13ccb95ed083397c5bff1e31ae1631b1091/proxy/src/console/provider.rs#L85-L90