introduce bmap support

Add the --bmap argument to take in a bmap file.

For example:
./fls from-url -k --debug --bmap autosd.bmap https://download.autosd.sig.cento
s.org/AutoSD-9/nightly/TI/auto-osbuild-am69sk-autosd9-qa-regular-aarch64-2154005
649.73882b72.raw.xz /dev/mmcblk1

This can arguably improve write performance

Signed-off-by: Benny Zlotnik <[email protected]>
Assisted-by: claude-sonnet-4.5

Author: bennyz