erofs-utils: tar: support ddtaridx format informally

erofs-utils: tar: support ddtaridx format informally

`ddtaridx` is a customized tar meta-only format implemented in
Alibaba's OverlayBD project [1].

Please don't use it externally if you have no idea of this except for
the OverlayBD project.  It will be removed if some better way exists.

[1] https://github.com/containerd/overlaybd

Cc: Yifan Yuan <tuji.yyf@alibaba-inc.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240713064028.4134602-1-hsiangkao@linux.alibaba.com