NUT-20: deterministic quote locking key derivation

[d4rp4t]

Stacked on feat/deterministic-p2pk (upstream PR Kukks#33), so the diff here is the one commit on top of it. Retarget to master once Kukks#33 lands.

Adds the derivation from cashubtc/nuts#373:

m/129373'/20'/0'/0'/{counter}

Verified against the vectors in tests/20-test.md (same mnemonic as the NUT-13 ones), plus a test that quote keys and P2PK keys never collide, since they differ only in the account index.

The counter is its own DerivationPurpose.MintQuoteLock, which the spec calls out as independent from the NUT-13 keyset counters.

Why it matters

Today a NUT-20 lock uses a key the caller generates and has to store somewhere. If it is lost, so is any quote that was locked but not yet minted — the ecash cannot be claimed. Deriving from the seed makes those quotes recoverable on restore.

WithDeterministicPubkey() locks the quote to the derived key and the mint handler signs the request itself, so neither WithPubkey nor SignWithPrivkey is needed:

var quote = await wallet
    .CreateMintQuote()
    .WithDeterministicPubkey()
    .WithAmount(1337)
    .ProcessAsyncBolt12();

var proofs = await quote.Mint();   // signed with the derived key

Requires a mnemonic and a counter implementing IDerivationCounter, both from the base branch.

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Unit tests: 116 passing. The integration test mints bolt12 end to end against cdk-mintd 0.17.3, checking that the pubkey the mint received is the derived one and that the counter advanced.