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).