Add NUMS point generation subcommand

[schoen]

We should have a subcommand to make NUMS points for use in internal keys, following the official BIP 0341 method.

We should also try to find ways to strongly discourage people from using any other form of number as a NUMS point (e.g. the default internal key selection mechanism should possibly only accept a representation of the salt/nonce for the BIP 0341 method, and make it inconvenient to use an internal key made by any other method, due to the greatly increased risk that that is part of a scam attempt).