[HTTP/3] Support on WinHttpHandler

[ManickaP]

Consider enabling H/3 for WinHttpHandler, see WINHTTP_PROTOCOL_FLAG_HTTP3.

Also find out if it's possible to turn on multiple connections for H/3 (or if it's the same option as for H/2 WINHTTP_OPTION_DISABLE_STREAM_QUEUE) and consider adding EnableMultipleHttp3Connections (same as EnableMultipleHttp2Connections).

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[stephentoub]

I assume WinHttpHandler uses msquic? If nothing else, this would be a good way to help validate the performance of the managed layer used by SocketsHttpHandler / System.Net.Quic, as the native quic implementation would be shared between the two.