RegisteredLink

###Example Usage
Using the RegisteredLink type, to create resources.

@RequestMapping("/")
public ResponseEntity get(RegisteredLinkProvider linkProvider){
    new Resource<String>(
        "Hello",
        linkProvider.getLinkWithSelfRel(Base),
        linkProvider.getLinkWithRel(Login),
        linkProvider.getLinkWithRel(User),
        linkProvider.getLinkWithRel(SecurityQuestions)
    );
    .....
    .....
}

### RegisteredLink
RegisteredLink acts as the registry for all links, providing a URI and 
a rel for all registered link. Uri is generated using Spring's 
ControllerLinkBuilder. LinkBuilder is supplied via closer to aid in testing
without a spring context.


public enum RegisteredLink {
    Login("login",
            () -> linkTo(methodOn(LoginController.class).login(null))),
    SecurityQuestions("security-questions",
            () -> linkTo(methodOn(SecurityQuestionsController.class).getSecurityQuestions())),
    CostEstimator("cost-estimator",
            () -> linkTo(methodOn(VitalsSSOController.class).ssoToVitals(null, null, null))),
    Base("base", 
            () -> linkTo(methodOn(BaseResourcesController.class).list(null, null))),

    User("registration", 
            () -> linkTo(methodOn(RegistrationController.class).register(null, null)));

    private final String relation;
    private final Supplier<LinkBuilder> linkBuilder;

    RegisteredLink(String relation, Supplier<LinkBuilder> linkBuilder) {
        this.relation = relation;
        this.linkBuilder = linkBuilder;
    }

    public String getRelation() {
        return relation;
    }

    public URI getUri() {
        return linkBuilder.get().toUri();
    }
}

### RegisterLinkProvider 
RegisteredLinkProvider acts a way to go from 
RegisteredLink -> SelfLink or RegisteredLink -> RelLink


public interface RegisteredLinkProvider {
    Link getLinkWithSelfRel(RegisteredLink registeredLink);
    Link getLinkWithRel(RegisteredLink registeredLink);
}

Two implementation of RegisterLinkProvider are provided,
RegisteredLinkProviderImpl and ProxiedRegisterLinkProvider.
The former provides links in a generic way and the later accommodates 
request arriving via proxy.

public class RegisteredLinkProviderImpl implements RegisteredLinkProvider {
    @Override
    public Link getLinkWithSelfRel(RegisteredLink registeredLink) {
        return new Link(registeredLink.getUri().toString())
                .withSelfRel();
    }
    @Override
    public Link getLinkWithRel(RegisteredLink registeredLink) {
        return new Link(registeredLink.getUri().toString())
                .withRel(registeredLink.getRelation());
    }
}


public class ProxiedRegisteredLinkProvider implements RegisteredLinkProvider {
    private String oauthHost;
    private String gatewayHost;

    public ProxiedRegisteredLinkProvider(String oauthHost, String gatewayHost) {
        this.oauthHost = oauthHost;
        this.gatewayHost = gatewayHost;
    }

    @Override
    public Link getLinkWithSelfRel(RegisteredLink registeredLink) {
        return getLink(registeredLink).withSelfRel();
    }

    @Override
    public Link getLinkWithRel(RegisteredLink registeredLink) {
        return getLink(registeredLink).withRel(registeredLink.getRelation());
    }

    private Link getLink(RegisteredLink registeredLink) {
        URIBuilder uriBuilder = new URIBuilder(registeredLink.getUri());
        switch (registeredLink) {
            case Login:
                uriBuilder.setHost(oauthHost);
                break;
            case User:
                uriBuilder.setHost(oauthHost);
            default:
                uriBuilder.setHost(gatewayHost);
        }
        return new Link(uriBuilder.toString());
    }
}

### Factory
Factory is provided to assist with RegisteredLinkProvider creations.

public class RegisteredLinkProviderFactory {

    private final String oauthHost;
    private final String gatewayHost;

    public RegisteredLinkProviderFactory(String oauthHost, String gatewayHost) {
        this.oauthHost = oauthHost;
        this.gatewayHost = gatewayHost;
    }

    public RegisteredLinkProvider proxiedRegisteredLinkProvider(){
        return new ProxiedRegisteredLinkProvider(oauthHost, gatewayHost);
    }

    public RegisteredLinkProvider standardRegisteredLinkProvider(){
        return new RegisteredLinkProviderImpl();
    }
}