Respect CancellationToken in HubConnection.StartAsync() (#10140)

src/SignalR/clients/csharp/Client.Core/src/HubConnection.cs

...alR/clients/csharp/Client/test/UnitTests/TestTransport.cs

...osoft.AspNetCore.Http.Connections.Client.netcoreapp3.0.cs

...soft.AspNetCore.Http.Connections.Client.netstandard2.0.cs

...ents/csharp/Http.Connections.Client/src/HttpConnection.cs

...csharp/Http.Connections.Client/src/Internal/ITransport.cs

...p.Connections.Client/src/Internal/LongPollingTransport.cs

...nections.Client/src/Internal/ServerSentEventsTransport.cs

...tp.Connections.Client/src/Internal/WebSocketsTransport.cs

src/SignalR/server/SignalR/test/EndToEndTests.cs