[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/ArmBot/include/subsystems/DriveSubsystem.h

@@ -6,7 +6,6 @@
 
 #include <frc/Encoder.h>
 #include <frc/drive/DifferentialDrive.h>
-#include <frc/motorcontrol/MotorControllerGroup.h>
 #include <frc/motorcontrol/PWMSparkMax.h>
 #include <frc2/command/SubsystemBase.h>
 
@@ -75,14 +74,9 @@ class DriveSubsystem : public frc2::SubsystemBase {
   frc::PWMSparkMax m_right1;
   frc::PWMSparkMax m_right2;
 
-  // The motors on the left side of the drive
-  frc::MotorControllerGroup m_leftMotors{m_left1, m_left2};
-
-  // The motors on the right side of the drive
-  frc::MotorControllerGroup m_rightMotors{m_right1, m_right2};
-
   // The robot's drive
-  frc::DifferentialDrive m_drive{m_leftMotors, m_rightMotors};
+  frc::DifferentialDrive m_drive{[&](double output) { m_left1.Set(output); },
+                                 [&](double output) { m_right1.Set(output); }};
 
   // The left-side drive encoder
   frc::Encoder m_leftEncoder;