libsnapshot: Refactor compress function

Move compress() function to a dedicated file.
This is in preparation for variable block size
compression and async writes.

No change in functional logic.

Bug: 254188450
Test: Full OTA Pixel
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I7681e9a4d884eac1ccbf8adeb1fc6bd1a9fedfa6