Allow the FileFd to use an external Compressor to uncompress a given file

Allow the FileFd to use an external Compressor to uncompress a given file
internally so that it is exported and can be used like a "normal"
uncompressed file with FileFd

This allows us to hide th zlib usage in the implementation and use gzip
instead if we don't have zlib builtin (the same for other compressors).

The code includes quiet a few FIXME's so while all tests are working
it shouldn't be used just yet outside of libapt as it might break.