[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/MecanumControllerCommand/include/subsystems/DriveSubsystem.h

@@ -148,7 +148,10 @@ class DriveSubsystem : public frc2::SubsystemBase {
   frc::PWMSparkMax m_rearRight;
 
   // The robot's drive
-  frc::MecanumDrive m_drive{m_frontLeft, m_rearLeft, m_frontRight, m_rearRight};
+  frc::MecanumDrive m_drive{[&](double output) { m_frontLeft.Set(output); },
+                            [&](double output) { m_rearLeft.Set(output); },
+                            [&](double output) { m_frontRight.Set(output); },
+                            [&](double output) { m_rearRight.Set(output); }};
 
   // The front-left-side drive encoder
   frc::Encoder m_frontLeftEncoder;