feat: support passing signingEndpoint to getSignedUrl in file operations

Author: thiyaguk09

handwritten/storage/src/file.ts

@@ -3236,6 +3236,7 @@ class File extends ServiceObject<File, FileMetadata> {
       contentMd5: cfg.contentMd5,
       contentType: cfg.contentType,
       host: cfg.host,
+      signingEndpoint: cfg.signingEndpoint,
     };
 
     if (cfg.cname) {

handwritten/storage/test/file.ts

@@ -3794,11 +3794,30 @@ describe('File', () => {
           contentType: config.contentType,
           cname: CNAME,
           virtualHostedStyle: true,
+          signingEndpoint: undefined,
         });
         done();
       });
     });
 
+    it('should pass signingEndpoint to URLSigner', done => {
+      const signingEndpoint = 'https://my-endpoint.com';
+      const config = {
+        signingEndpoint,
+        ...SIGNED_URL_CONFIG,
+      };
+
+      file.getSignedUrl(config, (err: Error | null) => {
+        assert.ifError(err);
+        const getSignedUrlArgs = signerGetSignedUrlStub.getCall(0).args;
+        assert.strictEqual(
+          getSignedUrlArgs[0]['signingEndpoint'],
+          signingEndpoint
+        );
+        done();
+      });
+    });
+
     it('should add "x-goog-resumable: start" header if action is resumable', done => {
       SIGNED_URL_CONFIG.action = 'resumable';
       SIGNED_URL_CONFIG.extensionHeaders = {