Spring Security OpenSamlAuthenticationRequestFactory should use OpenSAML directly

OpenSamlAuthenticationRequestFactory leverages a package-private class OpenSamlImplementation to remove boilerplate from its own class.

However, this boilerplate is only removed for OpenSamlAuthenticationRequestFactory and for no other class. For example, OpenSamlImplementation has a signAuthRequest method that's only called by OpenSamlAuthenticationRequestFactory.

Since these methods are only used by OpenSamlAuthenticationRequestFactory, they should be inlined. They can always be re-extracted should the need arise in the future.