watchtower: introduce CommitmentType

In this commit a new enum, CommitmentType, is introduced and initially
there are 3 CommitmentTypes: Legacy, LegacyTweakless and Anchor.

Then, various methods are added to `CommitmentType`. This allows us to
remove a bunch of "if-else" chains from the `wtclient` and `lookout`
code. This will also make things easier to extend when a new commitment
type (like Taproot) is added.

watchtower/blob/type.go

@@ -3,6 +3,8 @@ package blob
 import (
 	"fmt"
 	"strings"
+
+	"github.com/lightningnetwork/lnd/channeldb"
 )
 
 // Flag represents a specify option that can be present in a Type.
@@ -81,6 +83,27 @@ func (t Type) Identifier() (string, error) {
 	}
 }
 
+// CommitmentType returns the appropriate CommitmentType for the given blob Type
+// and channel type.
+func (t Type) CommitmentType(chanType *channeldb.ChannelType) (CommitmentType,
+	error) {
+
+	switch {
+	case t.Has(FlagAnchorChannel):
+		return AnchorCommitment, nil
+
+	case t.Has(FlagCommitOutputs):
+		if chanType != nil && chanType.IsTweakless() {
+			return LegacyTweaklessCommitment, nil
+		}
+
+		return LegacyCommitment, nil
+
+	default:
+		return 0, ErrUnknownBlobType
+	}
+}
+
 // Has returns true if the Type has the passed flag enabled.
 func (t Type) Has(flag Flag) bool {
 	return Flag(t)&flag == flag