Draft: [EXPERIMENT] begin adding "async" versions of trait/impls

[nardoor]

This is an experimentation branch. The way I duplicated code to """quickly""" have an async impl is a terrible way to do it. To me full code duplication is never a good option.

My goal is to reach a state of rustic/rustic_core/rustic_backend to experiment on the async implementations.

This might be a step towards a long term architectural change that could solve issues like rustic-rs/rustic#1181 and possibly enhance performances? (yet to be measured^^)

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Do you find interesting that I keep on going with this experimentation?