erofs-utils: dump: Add --cat flag to show file contents

erofs-utils: dump: Add --cat flag to show file contents

This patch adds a new '--cat' flag to the 'dump.erofs' command. When
used it will write to the standard output the content of the file
indicated by the '--path' or '--nid' options. For example, if there is a
'/mydir/myfile.txt' file containg the text 'mytext':

    $ dump.erofs --cat --path=/mydir/myfile.txt myimage.erofs
    mytext

Signed-off-by: Juan Hernandez <jhernand@redhat.com>
Link: https://lore.kernel.org/r/20250109105611.178398-1-jhernand@redhat.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>