watchtower: convert JusticeKit to interface

In this commit, we convert the `JusticeKit` struct to an interface.
Then, we add two implementations of that interface:
1) The `legacyJusticeKit` which implements all the methods of
   `JusticeKit`
2) The `anchorJusticKit` which wraps the `legacyJusticeKit` and just
   re-implements the `ToRemoteOutputSpendInfo` method since.

watchtower/wtdb/client_db_test.go

@@ -1201,7 +1201,10 @@ func randCommittedUpdateForChannel(t *testing.T, chanID lnwire.ChannelID,
 	_, err := io.ReadFull(crand.Reader, hint[:])
 	require.NoError(t, err)
 
-	encBlob := make([]byte, blob.Size(blob.FlagCommitOutputs.Type()))
+	kit, err := blob.AnchorCommitment.EmptyJusticeKit()
+	require.NoError(t, err)
+
+	encBlob := make([]byte, blob.Size(kit))
 	_, err = io.ReadFull(crand.Reader, encBlob)
 	require.NoError(t, err)
 
@@ -1229,7 +1232,10 @@ func randCommittedUpdateForChanWithHeight(t *testing.T, chanID lnwire.ChannelID,
 	_, err := io.ReadFull(crand.Reader, hint[:])
 	require.NoError(t, err)
 
-	encBlob := make([]byte, blob.Size(blob.FlagCommitOutputs.Type()))
+	kit, err := blob.AnchorCommitment.EmptyJusticeKit()
+	require.NoError(t, err)
+
+	encBlob := make([]byte, blob.Size(kit))
 	_, err = io.ReadFull(crand.Reader, encBlob)
 	require.NoError(t, err)