URLImageFileStore.init(fileIndex:) now public

[sergey-ilyin]

This patch allows to customize path to file storage:

       ```
         let fileIndexConfiguration = FileIndex.Configuration(
              name: "URLImage",
              filesDirectoryName: "images",
              directoryURL: /* OUR PATH */
        )
        let fileIndex = FileIndex( configuration: fileIndexConfiguration )

        let urlImageService = URLImageService(
              fileStore: URLImageFileStore( fileIndex: fileIndex ), // <-- this now public, otherwise will not compile
              inMemoryStore: URLImageInMemoryStore()
        )