Add HTTP/3 APIs

[scalablecory]

This adds APIs needed for HTTP/3.

struct System.Net.Security.SslApplicationProtocol
{
	public static readonly System.Net.Security.SslApplicationProtocol Http3;

	// Existing
	// public static readonly System.Net.Security.SslApplicationProtocol Http2;
	// public static readonly System.Net.Security.SslApplicationProtocol Http11;
}

class System.Net.HttpVersion
{
	public static readonly System.Version Version30;

	// Existing
	// public static readonly System.Version Version20;
	// public static readonly System.Version Version11;
	// public static readonly System.Version Version10;
}

[karelz]

Triage: We need to wait for QUIC spec to be finalized (its ALPN value).

[geoffkizer]

Spec is essentially done at this point, let's get these approved and implemented.

[terrajobst]

Video

• Looks good as proposed

namespace System.Net
{
    public class partial HttpVersion
    {
        // Existing
        // public static readonly Version Version10;
        // public static readonly Version Version11;
        // public static readonly Version Version20;
        public static readonly Version Version30;
    }
}
namespace System.Net.Security
{
    public partial struct SslApplicationProtocol
    {
        // Existing
        // public static readonly SslApplicationProtocol Http11;
        // public static readonly SslApplicationProtocol Http2;
        public static readonly SslApplicationProtocol Http3;
    }
}