add "gateway name" field set by auth blocks and an extban that matches it

[edk0]

so, for example, someone using the libera webchat would have "is using gateway: web.libera.chat" in their WHOIS, and would match $g:web.libera.chat

[glguy]

Preferences on the reply code we use for that?

[ncoevoet]

Could we also get a field in WHOX and a CAP for that ? ( onjoin )

[examknow]

Any thoughts on maximum length for gateway names?

[progval]

Bikeshedding: what about 'w' instead of 'g' as character? This seems pretty similar to Insp's ~$w extban https://docs.inspircd.org/3/modules/cgiirc/#extended-bans