fetchindex: make compression format configurable

Add INDEX_COMPRESSION_FORMAT as a variable a user can overwrite via
/etc/make.conf to allow the compression format used for INDEX to be
changed. Available compression formats are: bz2, xz and zst for now.

While here change the default compression format to be .xz (the
smallest), for environement where performance of unxz matters it can
switched to zst.

bz2 format is kept for compatibility with existing tooling.

In order to simplify the code, bsdcat(1) is now used for uncompressing
instead detecting the compression format manually and switch the
decompression tool manually

Note the bzip2 compression format is now considered as deprecated