add PSR6 cache adapter

[dmaicher]

Subject

I am targeting this branch, because its BC.

Closes https://github.com/sonata-project/cache/issues/50

Changelog

### Added
- Added `Psr6Cache` that allows using a PSR6 compliant cache implementation

Remaining questions

• should we deprecate all other cache adapters?
• what about CounterAdapterInterface? To me this does not even belong into this repository. Should we deprecate this fully? Or also add some PSR6 adapter for it? Where is this even used?
• How exactly do we continue moving away from this repository on BlockBundle and PageBundle?

[dmaicher]

We should discuss https://github.com/sonata-project/SonataBlockBundle/issues/663 first

[dmaicher]

Closing as we abandoned this project