Remove OpenSamlImplementation

[jzheaux]

OpenSamlImplementation is a package-private Spring Security class that exists to remove some of the boilerplate needed to configure and work with OpenSAML.

In effect, it does four things:

• Bootstraps OpenSAML with reasonable defaults
• Builds OpenSAML XMLObjects
• Marshals OpenSAML XMLObjects
• Unmarshals OpenSAML XMLObjects

While it was convenient to have in the early stages of creating saml2-service-provider, there's value in breaking it up into its individual components.

• - OpenSAML should get initialized as part of container lifecycle - OpenSAML should get initialized as part of container lifecycle #8772
• - OpenSamlAuthenticationProvider should use OpenSAML directly - OpenSamlAuthenticationProvider should use OpenSAML directly #8773
• - OpenSamlAuthenticationRequestFactory should use OpenSAML directly - OpenSamlAuthenticationRequestFactory should use OpenSAML directly #8774

The above plan requires minimal code duplication as the authentication provider is largely unmarshalling objects while the authentication request factory is marshalling them. Additionally, the marshalling support in OpenSamlImplementation is largely specific to AuthnRequest.