Allow service overrides

[alexb5dh]

This allows to override any of the default services:

var discv5Builder = new Discv5ProtocolBuilder(services)
    .WithServices(s => s
        .AddSingleton<IConnectionManager, MyConnectionManager>()
        .AddSingleton<IUdpConnection, MyConnection>()
    )
    //...

My use case is to replace IConnectionManager and IUdpConnection to integrate with DotNetty.

In current version this is possible, but very cumbersome, since call to AddDiscv5 replaces any of the user-registered variants.