With regards to Issue #1486 a discussion came up that the
way factories are implemented are questionable when thinking
about strong typing for the public kiwi interface. This
commit refactors the FileSystem factory to be a real factory
and to add type hints such that its use from an api perspective
is clear and enforced.
This commit adds the `squashfscompression` attribute in type element. It
can take `gzip`, `zstd`, `xz`, `lzo`, `lz4` or `none`. The default is `xz`.
Fixes#1315