OAuth2LoginConfigurer should discover OAuth2UserService beans

This would facilitate constructions like:

```java
void configure(HttpSecurity http) {
    http.
        oauth2Login();
}

@Bean
OAuth2UserService<OidcUserRequest, OidcUser> oidcUserService() {
    return new CustomOidcUserService();
}

@Bean
OAuth2UserService<OAuth2UserRequest, OAuth2UserService> oidcUserService() {
    return new CustomOAuth2UserService();
}
```