Introduce a new ext to allow downstream clients to have full control on the setup of the upstream connection, including things like pre-connection-registration SASL and account registration. Do not save the network before we get a RPL_WELCOME from upstream.

-> BOUNCER CONNECT ircs://irc.libera.chat
<- BOUNCER CONNECTED
# from this point messages are relayed to the upstream network
-> CAP LS 302
<- CAP LS :sasl=PLAIN message-tags
-> NICK yuki
-> USER yuki 0 * yuki
# ... more setup, e.g. SASL
-> CAP END
# at this point upstream sends RPL_WELCOME
# the bouncer saves the network in the DB
<- BOUNCER BIND 42
<- RPL_WELCOME
# from now on things work exactly like bouncer-networks does