Support Metadata in GridFSAdapter (#6660)

* Support Metadata in GridFSAdapter

* Useful for testing in the JS SDK
* Adds new endpoint to be used with `Parse.File.getData`
* Allows file adapters to return tags as well as future data.

* fix tests

* Make getMetadata optional

* Revert "fix tests"

This reverts commit 7706da13c688027483974e854b5b24321fb070cd.

* improve coverage

src/Adapters/Files/FilesAdapter.js

@@ -88,6 +88,14 @@ export class FilesAdapter {
    * @returns {Promise} Data for byte range
    */
   // handleFileStream(filename: string, res: any, req: any, contentType: string): Promise
+
+  /** Responsible for retrieving metadata and tags
+   *
+   * @param {string} filename - the filename to retrieve metadata
+   *
+   * @return {Promise} a promise that should pass with metadata
+   */
+  // getMetadata(filename: string): Promise<any> {}
 }
 
 /**