watchtower: add new TaprootCommit Type and flag

This commit adds a new FlagTaprootChannel Flag which is then used to
construct a new blob Type: TypeAltruistTaprootCommit. New watchtower
feature bits are also added (4/5).

watchtower/wtpolicy/policy.go

@@ -125,6 +125,12 @@ func (p Policy) IsAnchorChannel() bool {
 	return p.TxPolicy.BlobType.IsAnchorChannel()
 }
 
+// IsTaprootChannel returns true if the session policy requires taproot
+// channels.
+func (p Policy) IsTaprootChannel() bool {
+	return p.TxPolicy.BlobType.IsTaprootChannel()
+}
+
 // Validate ensures that the policy satisfies some minimal correctness
 // constraints.
 func (p Policy) Validate() error {