OAuth2ResourceServerSpec should allow its authentication manager to be configurable

[jzheaux]

If a user wanted to supply a custom authentication manager for a Reactive OAuth2 Resource Server, then she would need to create the entire AuthenticationWebFilter from scratch.

Since on the Reactive side of Spring Security, authentication managers are typically separated by Authentication type, it makes sense to expose OAuth2 Resource Server's authentication manager to be configurable:

http
    .oauth2ResourceServer()
        .jwt()
            .authenticationManager(customAuthenticationManager())