Add support for psr cache interfaces (flix-tech#37)

* Add support for psr-6 with the CacheItemPoolAdapter
* Add support for psr-16 with the SimpleCacheAdapter
* The package guzzlehttp/psr7 from version 1.7 breaks the tests, so 
  constraint it at version <1.7

Thanks to @fcoedno for the contribution!

Update `flix-tech/avro-php` dependency (flix-tech#36)

Co-authored-by: @nick-zh
Co-authored-by: @tPl0ch

[DOC] Preparing docs for new major release 7.0.0 (flix-tech#31)

Fixes flix-tech#26

* [DOC] Preparing docs for new major release 7.0.0
* [DOC] Fix type in `README.md`

[FT] Use new 3.0 version of `flix-tech/avro-php` (flix-tech#24)

[CI] Use wikimedias Avro library (flix-tech#22)

[FX] Fixing dealing with exceptions wrapped in fulfilled promises (fl…

…ix-tech#15)

[RELEASE][HOTFIX] Correct Stream processing with newer Guzzle versions (

flix-tech#14)

* [FX] Also add throwing values in rejection callbacks

* [FX] Also add throwing values in rejection callbacks for CachedRegistry

* [FX] Use correct stream string access in `ExceptionMap`

[FX] Use correct stream string access in `ExceptionMap`

[FX] Also add throwing values in rejection callbacks for CachedRegistry

[FX] Also add throwing values in rejection callbacks