Add pay-to-falcon-key (P2FK) note types to the standardized note scripts

[bobbinth]

Currently, our standardized notes specify account ID as the note target. This works fine, but sometimes it may be desirable for the recipient not to reveal their account ID to the sender. In such cases, the recipient may provide a public key to the sender, and the sender could construct the script in such a way that only the owner of the public key could consume the note.

Since we currently support Falcon signatures, we could add two new note types to the set of standardized note scripts:

• P2FK - pay-to-Falcon-public-key.
• P2FKR - pay-to-Falcon-public-key with recall.

In the future, when we have efficient ECDSA signatures, we could expand this to P2EK and P2EKR notes etc.