funding+lnwallet: only blind tapscript root early in funding flow

In this commit, we modify the aux funding work flow slightly. We won't
be able to generate the full AuxFundingDesc until both sides has
sent+received funding params. So we'll now only attempt to bind the
tapscript root as soon as we send+recv the open_channel message.

We'll now also make sure that we pass the tapscript root all the way
down into the musig2 session creation.

lnwallet/chanfunding/psbt_assembler.go

@@ -10,6 +10,7 @@ import (
 	"github.com/btcsuite/btcd/btcutil"
 	"github.com/btcsuite/btcd/btcutil/psbt"
 	"github.com/btcsuite/btcd/chaincfg"
+	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/btcsuite/btcd/txscript"
 	"github.com/btcsuite/btcd/wire"
 	"github.com/lightningnetwork/lnd/fn"
@@ -164,6 +165,13 @@ func (i *PsbtIntent) BindKeys(localKey *keychain.KeyDescriptor,
 	i.State = PsbtOutputKnown
 }
 
+// BindTapscriptRoot takes an optional tapscript root and binds it to the
+// underlying funding intent. This only applies to musig2 channels, and will be
+// used to make the musig2 funding output.
+func (i *PsbtIntent) BindTapscriptRoot(root fn.Option[chainhash.Hash]) {
+	i.tapscriptRoot = root
+}
+
 // FundingParams returns the parameters that are necessary to start funding the
 // channel output this intent was created for. It returns the P2WSH funding
 // address, the exact funding amount and a PSBT packet that contains exactly one