Add frame builder, add possibility to smoothly change placeholder to RawPicture

[stepushchik-denis-gismart]

This changes are very helpful in creating a better UX. It is about this issue.

Usage example:

          frameBuilder: (_, Widget child, PictureInfo? pictureInfo) {
            return AnimatedSwitcher(
              duration: const Duration(microseconds: 300),
              child: pictureInfo != null
                  ? child
                  : somePlaceholderWidget,
            );
          },