feat(sdk-core): add BUILD_ONLY flag to fanoutUnspents

TICKET: BTC-1527

Author: kcheng-bitgo

modules/bitgo/test/v2/unit/wallet.ts

@@ -1513,6 +1513,21 @@ describe('V2 Wallet:', function () {
 
       response.isDone().should.be.true();
     });
+
+    it('should only build tx (not sign/send) while fanning out unspents', async function () {
+      const path = `/api/v2/${wallet.coin()}/wallet/${wallet.id()}/fanoutUnspents`;
+      const response = nock(bgUrl).persist().post(path, _.matches({ unspents })).reply(200);
+
+      const unusedNocks = nock(bgUrl);
+      unusedNocks.get(`/api/v2/${wallet.coin()}/key/${wallet.keyIds()[0]}`).reply(200);
+      unusedNocks.post(`/api/v2/${wallet.coin()}/wallet/${wallet.id()}/tx/send`).reply(200);
+
+      await wallet.fanoutUnspents({ address, unspents }, ManageUnspentsOptions.BUILD_ONLY);
+
+      response.isDone().should.be.true();
+      unusedNocks.pendingMocks().length.should.eql(2);
+      nock.cleanAll();
+    });
   });
 
   describe('manage unspents', function () {

modules/sdk-core/src/bitgo/wallet/wallet.ts

@@ -815,9 +815,14 @@ export class Wallet implements IWallet {
    * @param {Number} params.maxFeeRate - The max limit for a fee rate in satoshis/kB
    * @param {Number} params.maxNumInputsToUse - the number of unspents you want to use in the transaction
    * @param {Number} params.numUnspentsToMake - the number of new unspents to make
+   *
+   * @param {ManageUnspentsOptions} option - flag to toggle build and send or build only
    */
-  async fanoutUnspents(params: FanoutUnspentsOptions = {}): Promise<unknown> {
-    return this.manageUnspents('fanout', params);
+  async fanoutUnspents(
+    params: FanoutUnspentsOptions = {},
+    option = ManageUnspentsOptions.BUILD_SIGN_SEND
+  ): Promise<unknown> {
+    return this.manageUnspents('fanout', params, option);
   }
 
   /**