Allow interfaces for instance-based remoting

[Jand42]

Allow using instance method remoting using interface types instead of just (usually abstract) classes. (See "Server-side customization" in https://developers.websharper.com/docs/v4.x/cs/remoting)

    public interface IRemotingHandler
    {
        [Remote]
        Task<MyResult> Handle(MyCommand command);
    }

This was possible in F# with WebSharper 3.0 but were removed from 4.0 because interfaces getting tight semantics for client-side use, but could be reintroduced with some metadata and compiler changes.