[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/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); }};
 };