[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/DifferentialDrivePoseEstimator/include/Drivetrain.h

@@ -21,7 +21,6 @@
 #include <frc/geometry/Quaternion.h>
 #include <frc/geometry/Transform3d.h>
 #include <frc/kinematics/DifferentialDriveKinematics.h>
-#include <frc/motorcontrol/MotorControllerGroup.h>
 #include <frc/motorcontrol/PWMSparkMax.h>
 #include <frc/simulation/AnalogGyroSim.h>
 #include <frc/simulation/DifferentialDrivetrainSim.h>
@@ -140,9 +139,6 @@ class Drivetrain {
   frc::PWMSparkMax m_rightLeader{3};
   frc::PWMSparkMax m_rightFollower{4};
 
-  frc::MotorControllerGroup m_leftGroup{m_leftLeader, m_leftFollower};
-  frc::MotorControllerGroup m_rightGroup{m_rightLeader, m_rightFollower};
-
   frc::Encoder m_leftEncoder{0, 1};
   frc::Encoder m_rightEncoder{2, 3};