Add oauth2Client MockMvc Test Support

[jzheaux]

We should add test support for OAuth 2.0 Clients that stands independent from OAuth 2.0/OIDC Login test support.

this.mvc.perform(get("/")
        .with(oauth2Client("foo-registration-id")));

this.mvc.perform(get("/")
        .with(oauth2Client().clientRegistration(c -> c
                .registrationId("foo-registration-id")
        ));

this.mvc.perform(get("/")
        .with(oauth2Client().clientRegistration(fooRegistration));