Consider using ProcessingChain for authproc processing

[pradtke]

Our authproc processing in AuthProcService->processState() is missing some features from the main ProcessingChain. If an authproc needs to redirect the user somewhere (for consent, mfa, etc) then there is no easy way to resume the processing from the next filter. The ProcessingChain class used by SSP allows for resuming by storing the not-yet processed filter in the $state.

I think wrapping ProcessingChain from AuthProcService would allow us to use authproc filters that perform redirects.

[restena-sw]

I also have a use case for an interactive authproc (MFA with the simplesamlphp-module-webauthn). Being able to run interactive authprocs would be a very valuable addition.

[pradtke]

When I did similar work for the authoauth2 module it was not as complicated as I was expecting. Most of that PR is documentation and testing, and maybe a dozen lines for the authproc method.

[pradtke]

Should be part of the next major release. Functionality merged in #228