erofs-utils: lib: add erofs_get_configure()

erofs-utils: lib: add erofs_get_configure()

This adds `erofs_get_configure()` to get the global configuration
`cfg`.  It allows external entities to change the global configuration
through this helper, thereby controlling the EROFS mkfs process.

It is just a temporary helper for liberofs and it will be deprecated
in the future.  Don't rely on it too much.

Signed-off-by: Hongzhen Luo <hongzhen@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240627022741.3912785-1-hongzhen@linux.alibaba.com
[ Gao Xiang: minor commit message update. ]
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>