Unified Backups: Add support for customizing compression

Proposal

We should provide a combination of compression params. By default we will give the user good defaults but allow then to fine tune for desired effects.

An example of good defaults is to make things like the previous GZIP_RSYNCABLE=yes the default.

We should also consider the approach of having "compression profiles" built in:

• faster (based on compression mechanism, we will use lower and faster compression levels + enable multithread when supported)
• balanced (the default: good compression / speed / resources utilization)
• more compression (smaller final files)
• low resources (whatever is required to use the least amount of resources)

In order to make compression customizable, we will need to perform some refactor on existing Backup codebase