feat(userAgent): allow for optional user agent to be provided (#1313)

Co-authored-by: Frank Natividad <frankyn@users.noreply.github.com>

Author: averikitsch

src/storage.ts

@@ -49,6 +49,7 @@ export interface StorageOptions extends GoogleAuthOptions {
   autoRetry?: boolean;
   maxRetries?: number;
   promise?: typeof Promise;
+  userAgent?: string;
   /**
    * The API endpoint of the service used to make requests.
    * Defaults to `storage.googleapis.com`.
@@ -370,6 +371,8 @@ export class Storage extends Service {
    *     attempted before returning the error.
    * @property {Constructor} [promise] Custom promise module to use instead of
    *     native Promises.
+   * @property {string} [userAgent] The value to be prepended to the User-Agent
+   *     header in API requests.
    */
   /**
    * Constructs the Storage client.