[wpimath] Add protobuf/struct for trivial types (#5935)

This implements de/serialization for the types that aren't templated (SwerveDriveKinematics) in C++ or where there is no trivial way to go round-trip (Splines) for the messages.

wpimath/src/main/proto/plant.proto

@@ -10,7 +10,4 @@ message ProtobufDCMotor {
   double stall_current = 3;
   double free_current = 4;
   double free_speed = 5;
-  double r = 6;
-  double kv = 7;
-  double kt = 8;
 }

wpimath/src/main/proto/trajectory.proto

@@ -15,6 +15,5 @@ message ProtobufTrajectoryState {
 }
 
 message ProtobufTrajectory {
-  double total_time = 1;
   repeated ProtobufTrajectoryState states = 2;
 }