Make custom fields repeated instead of optional

This way, one can send custom fields for all known implementations.
The client does not need to know which simulator it configures.
The simulator can take its designated custom message.

Author: g3force

internal/simctl/ssl_simulation_config.pb.go

@@ -56,12 +56,12 @@ message RobotSpecs {
     // Wheel angle configuration
     optional RobotWheelAngles wheel_angles = 13;
     // Custom robot spec for specific simulators (the protobuf files are managed by the simulators)
-    optional google.protobuf.Any custom = 14;
+    repeated google.protobuf.Any custom = 14;
 }
 
 message RealismConfig {
     // Custom config for specific simulators (the protobuf files are managed by the simulators)
-    optional google.protobuf.Any custom = 1;
+    repeated google.protobuf.Any custom = 1;
 }
 
 // Change the simulator configuration