credentials: Move user impersonation to peer::get_identity()

To retrieve user credentials, EapHost provides us the interactive user's
token we can use to impersonate.

By doing the impersonation early in peer::get_identity(), we don't need
to pass the token down the lower methods. This is rather a
simplification than a performance optimization.

Signed-off-by: Simon Rozman <simon@rozman.si>

lib/EAPBase/include/Config.h

@@ -308,6 +308,8 @@ namespace eap
         ///
         /// Generates public identity using current configuration and given credentials
         ///
+        /// Must be called in the connecting user context.
+        ///
         std::wstring get_public_identity(const credentials &cred) const;
 
     public: