Backport #35198 by @lunny
Fix#31113
After #22385 introduced LFS GC, it never worked due to a bug in the INI
library: fields in structs embedded more than one level deep are not
populated from the INI file.
This PR fixes the issue by replacing the multi-level embedded struct
with a single-level struct for parsing the cron.gc_lfs configuration.
Added a new test for retrieving cron settings to demonstrate the bug in
the INI package.
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
