Add nthKey support

[Sjors]

This is a very rough draft pull request. I have no experience with and no tooling for Java application at the moment, so this won't even compile.

The nthKey iOs app can be used a signer in a multisig setup. So far I've only tested it with Specter, so - if you're interested in adding this - we may have to iron out some compatibility issues.

The public key announce format it uses is similar to ColdCard, but using xpub instead of zpub, e.g.:

{"ExtPubKey":"xpub6E...","AccountKeyPath":"48h\/0h\/0h\/2h","MasterFingerprint":"00000000"}

(see also BlueWallet issue BlueWallet/BlueWallet#3653)

The wallet export is the same as Specter:

{ "label": "Wallet McWalletface", "blockheight": 660156, "descriptor": "wsh(sortedmulti(2,[....,...\/0\/*))#00000000" }

Basic characteristics of the app:

• keys can be imported using JSON or QR
• wallet metadata can be exported using JSON or QR (it can scan the Specter backup PDF)
• testnet, signet are free, mainnet is paid (unless you compile yourself from source)
• only sh-wpkh (native Segwit) is supported
• only multisig is supported (using ~BIP48)

Thumbnails generated with:

convert nthkey-orig.png -channel RGB -negate -resize 300x300 -background white -gravity center -extent 200x200 -type grayscale nthkey@3x.png
convert nthkey-orig.png -channel RGB -negate -resize 150x150 -background white -gravity center -extent 100x100 -type grayscale nthkey@2x.png
convert nthkey-orig.png -channel RGB -negate -resize 75x75 -background white -gravity center -extent 50x50 -type grayscale nthkey.png

[craigraw]

Apologies for the slow reply on this. I'd like Sparrow to support nthKey and this PR seems like a good start. I will try to find time to review more fully in the coming weeks.