NUT-XX: add quote offers

[zeugmaster]

NUT-XX: Quote Offers

This adds a standardised format for quote offers: a shareable string
(cquoteA..., CBOR + base64_urlsafe like NUT-18) with which an operator hands
a wallet a prepared mint or melt operation — e.g. a cash deposit or withdrawal
at a counter, prepared by an operator connected to the mint through a payment
processor. This complements custom payment methods (#382), where an operation is
often initiated by a counterparty rather than by the wallet.

Motivation

Displaying an existing quote (e.g. as a QR code) would be unsafe: NUT-04
requires quote ids to remain secret between user and mint, and anyone who scans
the code could front-run the mint operation.

How it works

A quote offer contains no quote at all. It encodes the mint URL, operation,
method, unit, amount, and a single-use ticket. The wallet claims the offer
by creating the quote itself via the regular NUT-04/NUT-05 endpoints,
referencing the ticket; first claim wins. No new endpoints are required.

• Mint offers must be claimed with a NUT-20 lock, so every quote is born
  locked to its owner's key. An attacker who scans the offer first can only
  create a quote locked to their own key; the payer's claim then fails visibly
  and the operator voids the ticket — theft degrades to denial of service.
• Melt offers are always asynchronous. The quote id is generated by the
  wallet's own request and never displayed, so it doubles as proof of who
  initiated the payment before a payout is handed over.

Also adds error codes 20010 and 20011 for unknown/expired and
already-claimed tickets.

Implementations

• CDK
• cashu-ts
• CashuSwift / macadamia