Previously, the `PreauthUserDetailsProvider` was only checking that a
valid Shib session existed by looking for the regular or HTTP Shib
session header. This check is now strengthened by validating the
correct header exists for the correct instance.

After a discussion about the project's design and requirements moving
forward, it was decided to refactor the project to remove
`AttributeMapper` classes and have `Provider` classes handle attribute
mapping as well as user loading. This also adds a third provider,
`PreauthHTTPUserDetailsProvider`, which can be used in environments
where Shibboleth attributes are sent as HTTP headers.