tools/cephfs_mirror: Don't use blockdiff on smaller files

tools/cephfs_mirror: Don't use blockdiff on smaller files

Introduce a new configuration option,
'cephfs_mirror_blockdiff_min_file_size', to control the minimum file
size above which block-level diff is used during CephFS mirroring.

Files smaller than the configured threshold are synchronized using
full file copy, while larger files attempt block-level delta sync.
This provides better flexibility across environments with varying
file size distributions and performance constraints.

The default value is set to 16_M (16 MiB). The value is read once
at beginning of every snapshot sync.

Fixes: https://tracker.ceph.com/issues/73452
Signed-off-by: Kotresh HR <khiremat@redhat.com>