[wpimath] Add methods to concatenate trajectories (#3139)

Co-authored-by: Tyler Veness <calcmogul@gmail.com>

wpimath/src/main/native/include/frc/trajectory/Trajectory.h

@@ -119,6 +119,16 @@ class Trajectory {
    */
   Trajectory RelativeTo(const Pose2d& pose);
 
+  /**
+   * Concatenates another trajectory to the current trajectory. The user is
+   * responsible for making sure that the end pose of this trajectory and the
+   * start pose of the other trajectory match (if that is the desired behavior).
+   *
+   * @param other The trajectory to concatenate.
+   * @return The concatenated trajectory.
+   */
+  Trajectory operator+(const Trajectory& other) const;
+
   /**
    * Returns the initial pose of the trajectory.
    *