[commands] C++ unique_ptr migration (#4319)

Add a CommandPtr with an internal unique_ptr to enable not needing to move the underlying classes, which is error-prone due to the potential for lambda captures.

wpilibcExamples/src/main/cpp/examples/Frisbeebot/cpp/RobotContainer.cpp

@@ -47,5 +47,5 @@ void RobotContainer::ConfigureButtonBindings() {
 
 frc2::Command* RobotContainer::GetAutonomousCommand() {
   // Runs the chosen command in autonomous
-  return &m_autonomousCommand;
+  return m_autonomousCommand.get();
 }

wpilibcExamples/src/main/cpp/examples/Frisbeebot/include/RobotContainer.h

@@ -45,7 +45,7 @@ class RobotContainer {
   ShooterSubsystem m_shooter;
 
   // A simple autonomous routine that shoots the loaded frisbees
-  frc2::SequentialCommandGroup m_autonomousCommand =
+  frc2::CommandPtr m_autonomousCommand =
       frc2::SequentialCommandGroup{
           // Start the command by spinning up the shooter...
           frc2::InstantCommand([this] { m_shooter.Enable(); }, {&m_shooter}),