[SignalR] Replace Single(connectionId) with Client(connectionId) and Caller

[halter73]

We recently discovered can add new Caller and Client members without breaking existing IHubClients and IHubCallerClients implementations unless they're being used for client results which never worked before .NET 7 making the change both non-binary and non-source breaking[1].

Before

public class MyHub : Hub
{
    public async Task BroadcastCallerResult()
    {
        var num = await Clients.Single(Context.ConnectionId).InvokeAsync<int>("GetNum");
        await Clients.Others.SendAsync("BroadcastNum", num);
    }
}

After

public class MyHub : Hub
{
    public async Task BroadcastCallerResult()
    {
        var num = await Clients.Caller.InvokeAsync<int>("GetNum");
        await Clients.Others.SendAsync("BroadcastNum", num);
    }
}

Fixes #41994

1. TODO Still manually test with a custom IHubClients implementation built against .NET 6.

[BrennanConroy]

1. Still manually test with a custom IHubClients implementation built against .NET 6.

Made a 6.0 app and published it, then ran the published app against a custom 7.0 runtime with these changes. Nothing broke as expected. Below is the rough code used.

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddSingleton(typeof(IHubContext<>), typeof(MyHubContext<>));
builder.Services.AddSignalR();
var app = builder.Build();

app.MapGet("/custom", (IHubContext<MyHub> c, [FromQuery] string id) =>
{
    c.Clients.Client(id).SendAsync("ReceiveMessage", "test");
});

app.MapHub<MyHub>("/hub");

app.Run();

public class MyHub : Hub { }
public class MyHubContext<THub> : IHubContext<THub> where THub : Hub
{
    private readonly HubLifetimeManager<THub> _lm;
    public MyHubContext(HubLifetimeManager<THub> lm)
    {
        _lm = lm;
    }

    public IHubClients Clients => new HubClient<THub>(_lm);

    public IGroupManager Groups => throw new NotImplementedException();
}

internal class HubClient<THub> : IHubClients where THub : Hub
{
    private readonly HubLifetimeManager<THub> _lm;
    public HubClient(HubLifetimeManager<THub> lm)
    {
        _lm = lm;
    }

    public IClientProxy Client(string connectionId)
    {
        return new SingleClientProxy(_lm, connectionId);
    }

    // all the interface methods with NotImplementedException, removed for brevity

    internal sealed class SingleClientProxy : IClientProxy
    {
        private readonly string _connectionId;
        private readonly HubLifetimeManager<THub> _lifetimeManager;

        public SingleClientProxy(HubLifetimeManager<THub> lifetimeManager, string connectionId)
        {
            _lifetimeManager = lifetimeManager;
            _connectionId = connectionId;
        }

        public Task SendCoreAsync(string method, object?[] args, CancellationToken cancellationToken = default)
        {
            return _lifetimeManager.SendConnectionAsync(_connectionId, method, args, cancellationToken);
        }
    }
}