Generic promise

see #49

Php 8 support

Remove support for deprecated php version and add support for php 8.

Added adapter for Symfony HttpClient

See #37

Better errors managment

See #32 #34 #35
Fixed some edge cases introduced in previous releases.

Improvements related to error management

See #27 #28 #29 for more technical details.
Related documentation has been updated in #30.

Minor documentation improvements #26 #31

First official release

Merge pull request #25 from M6Web/feature/throw-from-unwatched-promise

Throwing exceptions from background asynchronous functions