tProxy needs a way to deal with TLV `UserIdentity` bigger than 32 bytes

[plebhash]

#295 reported an issue where tProxy was panicking whenever the Sv1 username (which later becomes TLV UserIdentity) is bigger than 32 bytes

#395 applied some temporary hotpatch, with a few compromises:

• we do a lossy truncation of the stored user_identity copy of the username field of Sv1 mining.authorize, leaving room for collisions
• this truncation happens with a warning, which is triggered even if extension 0x0002 is not active, leaving room for confusing UX

These compromises should be replaced with more sustainable long-term solutions.

We should also define clearer boundaries for:

• the config user_identity used for OpenExtendedMiningChannel
• the Sv1 worker name from mining.authorize
• the 32-byte TLV UserIdentity used by extension 0x0002

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relevant note: #295 (comment)