extend PT args size limit

Currently the maximum length of arguments on a bridge line is 510bytes, as those are passed in the username and password fields of the SOCKS5 connection. We have already hit this limit with snowflake (tpo/applications/tor-browser-build#40665 (closed)).

One proposal would to define a SOCKS5 METHOD (section 3 of RFC1928 different that 'username/password' (0x02) for it. Some years ago this was discussed and proposed to use 0x80 (RESERVED FOR PRIVATE METHODS): tpo/anti-censorship/pluggable-transports/trac#10671 (comment 2604090)

There is a PT spec around that proposes using 0x9 (undefined in the SOCKS5 RFC): https://github.com/Pluggable-Transports/Pluggable-Transports-spec/blob/main/releases/PTSpecV3.0/Pluggable%20Transport%20Specification%20v3.0%20-%20Dispatcher%20IPC%20Interface%20v3.0.md#14-pluggable-pt-client-per-connection-arguments

But AFAIK there is no implementation of any of those, so I guess we are free to define what we find more useful here. Any better proposals?