make cosa diff support diffing artifact sizes between builds

[dustymabe]

I wrote diffbuildsizes.py.txt a while ago when we switched to OSBuild to try to verify nothing dramatic was happening when we went from one build tool to the next. This was before cosa diff existed. Let's somehow incorporate this into cosa diff. It just uses the sizes reported in meta.json so it's pretty lightweight.

An example of the output looks like:

    platform     | compression  | 40.20240131.91.0 | 40.20240201.91.0
---------------------------------------------------------------------
qemu             | compressed   | 663MiB           | 848MiB          
qemu             | uncompressed | 1577MiB          | 876MiB          
metal4k          | compressed   | 661MiB           | 670MiB          
metal4k          | uncompressed | 2458MiB          | 2458MiB         
metal            | compressed   | 663MiB           | 698MiB          
metal            | uncompressed | 2458MiB          | 2458MiB