Refactor uw-php-security to use 'Provider' classes

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.