Split private gateways under tenant in uplink / downlink.

This makes it possible to share uplink data with other tenants, but do
not allow other tenants to use these gateways for downlinks.

api/python/proto/chirpstack-api/internal/internal.proto

@@ -196,6 +196,15 @@ message DeviceGatewayRxInfoItem {
 
     // Context blob.
     bytes context = 6;
+
+    // Gateway is private (uplink).
+    bool is_private_up = 7;
+
+    // Gateway is private (downlink).
+    bool is_private_down = 8;
+
+    // Tenant ID (UUID).
+    bytes tenant_id = 9;
 }
 
 message DownlinkFrame {