Signmessage

[Lagrang3]

Add a new rpc called signmessagewithkey that can be used to sign messages using any key
from our wallet. You cannot directly select the key to use, but instead you provide a bitcoin address
and the wallet will figure out which of our keys corresponds to that bitcoin address.

Solves issue #8199

TODO:

• add verification command
• add tests
• add documentation

[Lagrang3]

There is a variety of signature schemes in the market.
But it seems that Electrum's is somehow accepted as a standard and that seems to be
what Ocean accepts as well.

Therefore I implemented Electrum's signature scheme:

signature = base64(SigRec(SHA256(SHA256(
    "\x18Bitcoin Signed Message:\n" + var_int(len(message)) + message
))))

I checked against sparrow wallet and the verification succeeds.

$ l1-cli signmessagewithkey "some_string" "bcrt1q30zh8czhw4vzcv78t3ddv7zthtsw7w6wpgqz6z"
{
   "bech32": "bcrt1q30zh8czhw4vzcv78t3ddv7zthtsw7w6wpgqz6z",
   "keyidx": 1,
   "pubkey": "02fa14da5717ee05cd1eaf31ff871b8278f0b5dbe3a77ef3bf607c81b7391febe2",
   "signature": "204b7748da7641b218e0e3369f10c12bfd42fa39ed87aa27006499b8f5238b627b22f1d2c3f11de9b577a313760a9c5009fb91ed3d05480e782662559485a78fd7",
   "base64": "IEt3SNp2QbIY4OM2nxDBK/1C+jnth6onAGSZuPUji2J7IvHSw/Ed6bV3oxN2CpxQCfuR7T0FSA54JmJVlIWnj9c="
}

[Lagrang3]

In the future if it becomes necessary one could add a flag to select the signature scheme of choice.

Also for reviewers: I was tempted to make hsmd sign any message fromwire, adding more flexibility in
the signing format. But I didn't because I am not sure how much of a security vulnerability that could represent.
As a matter of fact in the comments of handle_sign_message in hsmd/libhsmd.c one can read
that prefixing the message save us from signing unwanted data.

[luke-jr]

Electrum-style is probably the worst of the options accepted by OCEAN fwiw

BIP 322 will be required if you need to support Taproot in the future.

BIP 137 would be probably trivial to add here (just a version byte change)