lnwallet: update the SignDescriptor struct to use keychain.KeyDescriptor
lnwallet: update the SignDescriptor struct to use keychain.KeyDescriptor

In this commit, we update the SignDescriptor struct to instead use the
keychain.KeyDescriptor struct inplace of a raw public key. We do this
as this allows for a recovery mode, where the caller doesn’t know what
key was used exactly, but remembers the precise KeyDescriptor. A
stateless signer is still able to re-derive the proper key, as it’s
given the full path.

aakselrod/lnd,LightningNetwork/lnd,LightningNetwork/lnd,aakselrod/lnd