Add a download test

[davidfowl]

We need a test to verify how fast ASP.NET Core can transfer data over the network. I'm talking about downloading large files in the GB ish streamed from server to client. We should measure the latency and transfer rate.

cc @sebastienros

[sebastienros]

We've done that already with static files of different size. I an find an issue with the results. And yarp has some benchmarks that go to the max 40Gbps limit.

But maybe you mean on a single connection. Also what does latency mean in this case? I'd be prone to think we need to have some automated benchmark (not continuous as in every day, less frequent) that would compare the result to other stacks, including served from nginx.