lnwallet: integrate new taproot channels into internal funding flow

In this commit, we build on all the prior commits and integrate the new
taproot channels into the existing internal funding flow. Along the way,
we do some refactoring to unify things like signing and verifying
incoming commitment transaction signatures.

For our local nonce, we use the existing functional option type to
derive the nonce based on the initial shachain pre-image we'll use as
our revocation.

lnwallet/channel.go

@@ -2586,7 +2586,6 @@ func createBreachRetribution(revokedLog *channeldb.RevocationLog,
 
 			// Read the amounts from the breach transaction.
 			theirAmt = spendTx.TxOut[theirOutpoint.Index].Value
-
 		} else {
 			// Otherwise, we check to see if the revocation log
 			// contains remote parties' output amount. Due to a