feat(quic): replace probe UDP data plane with QUIC datagrams

backend/shared/MoShiro.Measurement/Net/QuicAlpn.cs

@@ -0,0 +1,7 @@
+namespace MoShiro.Measurement;
+
+/// <summary>The application protocol negotiated by the Agent and Probe QUIC endpoints.</summary>
+internal static class QuicAlpn
+{
+    internal static readonly byte[] Value = "h3"u8.ToArray();
+}