SignerService: request signature using BIP32Derivation field

[louisinger]

TL;DR

• SignerService will fallback to input.BIP32Derivation[0] field to derivate the signing key if witnessUtxo.script is unknwown.
• Add a test case signing a simple pset without witnessUtxo.

Note that the fields fingerPrint and path are not used because Marina is mapping derivation path to script. Instead this PR uses a trick to re-compute the P2WPKH script from publicKey field. In other words your only need to set up a marina public public in the input to request a signature.

try it yourself: marina-0.5.4.zip

[jamesdorfman]

Great work, thank you. tACK 5888ff3ae1a50203274350402aee0edf71d487cd.

Marina successfully signed my PSET when I provided a Bip32Derivation.