NUT-11: Pay to Public Key (P2PK)

[KKA11010]

Additional context The NUT-11 describes Pay-to-Public-Key (P2PK) which is one kind of spending condition based on NUT-10's well-known Secret. Using P2PK, we can lock ecash tokens to a receiver's ECC public key and require a Schnorr signature with the corresponding private key to unlock the ecash. The spending condition is enforced by the mint.