Non-Generic UnaryResult

[mayuki]

This PR introduces non-generic UnaryResult.

Non-generic UnaryResult is the return type of a service method that does not return a value.
It replaces UnaryResult<Nil>, which is similar to Task and ValueTask.

Example

// Shared interface:
public interface IMyService : IService<IMyService>
{
    UnaryResult MethodAsync(int arg0);
}

// Server-side:
public class MyService : ServiceBase<IMyService>, IMyService
{
    public async UnaryResult MethodAsync(int arg0)
    {
        // Do something ...
        // The method does not return any value. (like void, ValueTask, Task)
    }
}

// Client-side:
await client.MethodAsync(1234);

Breaking changes

UnaryResult has changed from static class to struct.