Add interop tests to verify that Kestrel's HTTP/2 implementation works with HttpClient's implementation

[Tratcher]

There are not currently any functional test for the Http2 features. We accidentally disabled the entire feature when merging the https and core packages due to a missing TFM and didn't notice. aspnet/KestrelHttpServer#2224

The big blocker here is the lack of consistent client support. However, even if there's one client on one of the OS's then that would be a good start.

[linianhui]

Does client mean web browser or web server?

[Tratcher]

Browsers are clients, yes. System.Net.Http.HttpClient is the client we prefer for functional tests.

[linianhui]

@Tratcher But now System.Net.Http.HttpClient does not support http2. (￣▽￣)"

[benaadams]

HttpClient supports http2 on .NET Core for UWP, Linux dotnet/corefx#4756 and Windows dotnet/corefx#10948 (but not Full Framework as yet)

[Tratcher]

Note the Windows version restriction as well.
Windows 10 Anniversary release a.k.a Windows 10 Version 1607

[Tratcher]

I did some initial work in #2665 but only for Win10 WinHttpHandler. Linux and Mac have too many dependency issues, we may have to wait for SocketsHttpHandler.

[muratg]

The remaining test we'd like to add is using new HttpClient.