wallet: scaffold BIP86 addrtype

Add the UTXO_P2TR_BIP86 in preparation to add BIP86 wallet functions such as newaddr, listaddr etc. We also add a new index in the database for BIP86 as this is using a completely different derivation path and hsm_secret.

bitcoin/tx.c

@@ -940,6 +940,7 @@ size_t bitcoin_tx_input_witness_weight(enum utxotype utxotype)
 		/* In practice, these predate anchors, so: */
 		return 1 + 1 + bitcoin_tx_input_sig_weight();
 	case UTXO_P2TR:
+	case UTXO_P2TR_BIP86:
 		return 1 + 64;
 	}
 	abort();

bitcoin/tx.h

@@ -53,6 +53,8 @@ enum utxotype {
 	UTXO_P2WSH_FROM_CLOSE = 3,
 	/* "p2tr" addresses. */
 	UTXO_P2TR = 4,
+	/* "bip86" addresses (P2TR with BIP86 derivation). */
+	UTXO_P2TR_BIP86 = 5,
 };
 
 struct bitcoin_tx_output *new_tx_output(const tal_t *ctx,