lnwallet: modify btcwallet's SignOutputRaw method to get proper pubkey
lnwallet: modify btcwallet's SignOutputRaw method to get proper pubkey

This commit slightly modifies btcwallet’s SignOutputRaw method to work
properly in the case that the pkScript of the output being spent isn’t
one of the template pkScripts (p2pkh, multi-sig, etc). Rather than
examining the address, we now attempt to find the private key which
matches the public key passed within the sign descriptor.

aakselrod/lnd,ddustin/lnd,ddustin/lnd,AndrewSamokhvalov/lnd,aakselrod/lnd,LightningNetwork/lnd,bryanvu/lnd,LightningNetwork/lnd,AndrewSamokhvalov/lnd,bryanvu/lnd