Outbound http connections to destinations are configurable

[samsp-msft]

dotnet/runtime#987

[samsp-msft]

Runtime feature: Enables control of the version to prevent connections from being downgraded.

[Tratcher]

A partner requires this for use with IHttpProxy because they prefer HTTP/1.1 for their scenarios. We currently default to HTTP/2.

Proposal:
RequestProxyOptions is used with IHttpProxy and could also be exposed as per-cluster configuration.

reverse-proxy/src/ReverseProxy/Service/Proxy/RequestProxyOptions.cs

Lines 9 to 22 in 4bd161b

	internal class RequestProxyOptions
	{
	public Transforms Transforms { get; set; } = Transforms.Empty;
	/// <summary>
	/// The time allowed to send the request and receive the response headers. This may include
	/// the time needed to send the request body.
	/// </summary>
	public TimeSpan RequestTimeout { get; set; } = TimeSpan.FromSeconds(100);
	// Future:
	// ResponseBodyTimeout - The time allowed to receive the full response body. Default to infinite. Not applied to Upgraded requests or gRPC streams.
	// HttpVersion - Default to HTTP/2?
	// HttpVersionPolicy - Default to OrLower?

This would also address #319 (configurable timeouts).

Make the http version and version policy (5.0 only) available on the options and then use them when constructing requests in HttpProxy.

Should the version property be nullable to allow you to preserve the incoming request version?

[Tratcher]

The version policy should also be added to the RequestParametersTransformContext so it can be customized by transforms.

reverse-proxy/src/ReverseProxy/Service/RuntimeModel/Transforms/RequestParametersTransformContext.cs

Line 22 in 6311df4

public Version Version { get; set; }