Replace custom CloudEvents implementation with official sdk

[hendrikheil]

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Describe the feature

I think this library could benefit from using the official cloudevents php-sdk implementation. There has recently been work on an official implementation of cloudevents of a php-sdk. It looks like it could replace the current implementation just fine.

https://github.com/cloudevents/sdk-php

I'd be happy to contribute here and replace the implementation :)

Release Note

RELEASE NOTE: UPDATE switch to using cloudevents/php-sdk