SP redesign material

[Cantor, Scott]

As some very early code as started to emerge, and some questions have already come in, I wanted to get some early design material organized.

https://shibboleth.atlassian.net/wiki/spaces/DEV/pages/2981429255/Service+Provider+V4+Redesign

This has pre-existing notes and new material being developed as things evolve. I'll try to keep it up to date as work progresses so that eventually there's a foundation to build real API documentation from in the unlikely but not impossible event that somebody else wants to work on an agent for something outside of the initial Apache/IIS/FastCGI footprint.

-- Scott