Shibboleth idp v5: Add custom extension to AuthnRequest on SAML proxy to upstream idp

[Cantor, Scott]

The outbound interceptor feature does not run in the middle of proxying.

There's a property I never documented for injecting a Function into the outbound message handler sequence that runs before signing, I added a line for it to the reference in [1] in the table for SAML-specific properties.

The signature of the function is noted there and if you could manage to do all that, it's trivial in comparison.

There's also a more general hook for the rest of the message processing chains outside of proxying that's on the SAMLProfileConfiguration that's similar and again avoids the need for an interceptor flow.

-- Scott

[1] https://shibboleth.atlassian.net/wiki/spaces/IDP5/pages/3199505973/SAMLAuthnConfiguration