Refactor protobuf messages

[oswinso]

Currently, we have a giant RequestMessage and ResponseMessage that contains everything. I think we should break out and group the individual messages so that it's easier to manage, ie.

message RequestMessage {
  oneof message {
    SetPID settings = 0;
    MotorCommand command = 1;
  }
}

instead of

message RequestMessage {
    optional float p_l = 1;
    optional float p_r = 2;
    optional float i_l = 3;
    optional float i_r = 4;
    optional float d_l = 5;
    optional float d_r = 6;

    optional float speed_l = 7 [default = 0];
    optional float speed_r = 8 [default = 0];

    optional float kv_l = 9;
    optional float kv_r = 10;
}