Access principal name from servlet filter

[Tom Zeller]

> It's a lot of boilerplate and overhead, but the purpose of it is to create a lot of really bulletproof isolation between our stuff and your stuff that survives upgrades.

I started some docs regarding how-to-write-a-profile-intercept-flow
[1], if it's helpful I can add more.

[1] https://wiki.shibboleth.net/confluence/display/~tzeller@shibboleth.net/How+to+Write+a+Profile+Intercept+Flow