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.

Apologies for the big PR!

Please review: @ahoffmann

README.md

@@ -2,15 +2,15 @@
 
 uw-php-security is a PHP companion to [uw-spring-security](https://git.doit.wisc.edu/adi-ia/uw-spring-security) for Java. Its purpose is to ease development of PHP applications needing details about UW users provided through Shibboleth.
 
-Like uw-spring-security, uw-php-security provides a class called `UWUserDetails` for easily accessing common UW user attributes. This model is provided to applications through a `UserDetailsService`, and uw-php-security provides implementations suitable for both local and preauth (Shibboleth) environments.
+Like uw-spring-security, uw-php-security provides a class called `UWUserDetails` for easily accessing common UW user attributes. This model is provided to applications through a `UserDetailsProvider`, and uw-php-security provides implementations suitable for both local and preauth (Shibboleth) environments.
 
 ```php
 <?php
 
-use edu\wisc\doit\PreauthUserDetailsService; // or LocalUserDetailsService for local development
+use edu\wisc\doit\PreauthUserDetailsProvider; // or LocalUserDetailsProvider for local development
 ...
-$userDetailsService = new PreauthUserDetailsService();
-$user = $userDetailsService->loadUser();
+$userDetailsProvider = new PreauthUserDetailsProvider();
+$user = $userDetailsProvider->loadUser();
 if ($user == null) {
     // handle error
 }