[Dovecot] feature request: IMAP passdb prefetch

It'd be useful for us if the IMAP passdb could be used as a prefetch userdb.

The remote IMAP server could respond with something like

• OK key=value
• OK key=value SEQ OK [CAPABILITY ...] Logged in.

Or

• OK <some JSON key value pairs> SEQ OK [CAPABILITY ...] Logged in.

Would anyone else find this useful?