Add GZIP_EXT config variable to allow clean use of other compression tools

I plan to replace gzip by zstd for my ldap, mysql, pg and mongo dumps, because it is twice better and 3 times faster.

It is already possible to do it with GZIP=/usr/bin/zstd, but this PR allows to change the extensions from .gz to .zst.

I prefer not to rename the GZIP variable because we want to stay fully retro-compatible with old backupninja.conf and custom handlers.