Add plumbing for Diagnostics UI.

This is only compiled in if STEAMNETWORKINGSOCKETS_ENABLE_DIAGNOSTICSUI is
defined, which is only the case when running with Steam.  So no functional
change (is intended) for the opensource version of the code, just syncing
up with Steam.

P4:6570679,6570685,6570693,6570802,6570807,6570812

Author: zpostfacto

src/steamnetworkingsockets/clientlib/csteamnetworkingsockets.h

@@ -287,6 +287,18 @@ class CSteamNetworkingUtils : public IClientNetworkingUtils
 	// Reset this utils instance for testing
 	virtual void TEST_ResetSelf();
 
+	// Post a connection update message to the OS diagnostics
+	#ifdef STEAMNETWORKINGSOCKETS_ENABLE_DIAGNOSTICSUI
+		virtual void PostConnectionStateUpdateForDiagnosticsUI( ESteamNetworkingConnectionState eOldState, CSteamNetworkConnectionBase *pConnection, SteamNetworkingMicroseconds usecNow ) = 0;
+
+		// Desired update interval for connections, outside of state changes.
+		// The default is zero, which means no updates, not even for state changes.
+		// Derived classes that override PostConnectionStateUpdateForDiagnosticsUI must set this.
+		SteamNetworkingMicroseconds m_usecConnectionUpdateFrequency = 0;
+	#else
+		static constexpr SteamNetworkingMicroseconds m_usecConnectionUpdateFrequency = 0;
+	#endif
+
 	// Stubs if SDR not enabled
 #ifndef STEAMNETWORKINGSOCKETS_ENABLE_SDR
 	virtual ESteamNetworkingAvailability GetRelayNetworkStatus( SteamRelayNetworkStatus_t *pDetails ) override