[wpilib] Add functional interface equivalents to MotorController (#6053)

This does not deprecate any current functionality, but prepares the way for future deprecation.

The drive classes now accept void(double) functions, which makes them more flexible.

The C++ API ended up a bit more verbose, but the Java API is really concise with method references, which is >80% of our userbase. For example:

`DifferentialDrive drive = new DifferentialDrive(m_leftMotor::set, m_rightMotor::set);`

Lambdas can be passed to interoperate with vendor motor controller APIs that don't have e.g., set(double), so CTRE doesn't have to maintain their WPI_ classes anymore.

MotorControllerGroup was replaced with PWMMotorController.addFollower() for PWM motor controllers. Users of CAN motor controllers should use their vendor's follower functionality.

wpilibcExamples/src/main/cpp/examples/DriveDistanceOffboard/include/ExampleSmartMotorController.h

@@ -4,15 +4,13 @@
 
 #pragma once
 
-#include <frc/motorcontrol/MotorController.h>
-
 /**
  * A simplified stub class that simulates the API of a common "smart" motor
  * controller.
  *
  * <p>Has no actual functionality.
  */
-class ExampleSmartMotorController : public frc::MotorController {
+class ExampleSmartMotorController {
  public:
   enum PIDMode { kPosition, kVelocity, kMovementWitchcraft };
 
@@ -68,17 +66,17 @@ class ExampleSmartMotorController : public frc::MotorController {
    */
   void ResetEncoder() {}
 
-  void Set(double speed) override { m_value = speed; }
+  void Set(double speed) { m_value = speed; }
 
-  double Get() const override { return m_value; }
+  double Get() const { return m_value; }
 
-  void SetInverted(bool isInverted) override {}
+  void SetInverted(bool isInverted) {}
 
-  bool GetInverted() const override { return false; }
+  bool GetInverted() const { return false; }
 
-  void Disable() override {}
+  void Disable() {}
 
-  void StopMotor() override {}
+  void StopMotor() {}
 
  private:
   double m_value = 0.0;

wpilibcExamples/src/main/cpp/examples/DriveDistanceOffboard/include/subsystems/DriveSubsystem.h

@@ -83,5 +83,7 @@ class DriveSubsystem : public frc2::SubsystemBase {
   frc::SimpleMotorFeedforward<units::meters> m_feedforward;
 
   // The robot's drive
-  frc::DifferentialDrive m_drive{m_leftLeader, m_rightLeader};
+  frc::DifferentialDrive m_drive{
+      [&](double output) { m_leftLeader.Set(output); },
+      [&](double output) { m_rightLeader.Set(output); }};
 };