dx: throw exception if policy could not be found in policy registry

[Bl00D4NGEL]

This PR make the HasPolicies trait throw an exception if it can not find a policy for a subject with a (hopefully) useful exception message pointing the developer into a direction where or how they could fix the issue. Additionally I moved away from reflection classes in favor of built-in PHP methods to check for method existance and return value types.

In order to properly test these changes I went ahead and added phpunit as a dependency to add tests for this behaviour and added a GH actions definition (mostly copied from https://github.com/geekcell/container-facade/blob/main/.github/workflows/tests.yaml).

While this technically is a backwards compatibility break, in the previous version(s) of this bundle, if a policy was not found, the code would just error out completely because php cannot instantiate a reflection class of null.