feat(abstract-utxo): add encodeTransaction function

Add helper function to convert various transaction types to a Buffer

Issue: BTC-2806

Co-authored-by: llm-git <[email protected]>

Author: OttoAllmendinger

modules/abstract-utxo/src/transaction/decode.ts

@@ -57,3 +57,15 @@ export function decodePsbtWith(
     return fixedScriptWallet.BitGoPsbt.fromBytes(psbt, toNetworkName(network));
   }
 }
+
+export function encodeTransaction(
+  transaction: utxolib.bitgo.UtxoTransaction<bigint | number> | utxolib.bitgo.UtxoPsbt | fixedScriptWallet.BitGoPsbt
+): Buffer {
+  if (transaction instanceof utxolib.bitgo.UtxoTransaction) {
+    return transaction.toBuffer();
+  } else if (transaction instanceof utxolib.bitgo.UtxoPsbt) {
+    return transaction.toBuffer();
+  } else {
+    return Buffer.from(transaction.serialize());
+  }
+}