[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.

wpilibjExamples/src/main/java/edu/wpi/first/wpilibj/templates/romicommandbased/subsystems/RomiDrivetrain.java

@@ -24,7 +24,8 @@ public class RomiDrivetrain extends SubsystemBase {
   private final Encoder m_rightEncoder = new Encoder(6, 7);
 
   // Set up the differential drive controller
-  private final DifferentialDrive m_diffDrive = new DifferentialDrive(m_leftMotor, m_rightMotor);
+  private final DifferentialDrive m_diffDrive =
+      new DifferentialDrive(m_leftMotor::set, m_rightMotor::set);
 
   /** Creates a new RomiDrivetrain. */
   public RomiDrivetrain() {

wpilibjExamples/src/main/java/edu/wpi/first/wpilibj/templates/romieducational/RomiDrivetrain.java

@@ -23,7 +23,8 @@ public class RomiDrivetrain {
   private final Encoder m_rightEncoder = new Encoder(6, 7);
 
   // Set up the differential drive controller
-  private final DifferentialDrive m_diffDrive = new DifferentialDrive(m_leftMotor, m_rightMotor);
+  private final DifferentialDrive m_diffDrive =
+      new DifferentialDrive(m_leftMotor::set, m_rightMotor::set);
 
   /** Creates a new RomiDrivetrain. */
   public RomiDrivetrain() {

wpilibjExamples/src/main/java/edu/wpi/first/wpilibj/templates/romitimed/RomiDrivetrain.java

@@ -23,7 +23,8 @@ public class RomiDrivetrain {
   private final Encoder m_rightEncoder = new Encoder(6, 7);
 
   // Set up the differential drive controller
-  private final DifferentialDrive m_diffDrive = new DifferentialDrive(m_leftMotor, m_rightMotor);
+  private final DifferentialDrive m_diffDrive =
+      new DifferentialDrive(m_leftMotor::set, m_rightMotor::set);
 
   /** Creates a new RomiDrivetrain. */
   public RomiDrivetrain() {

wpilibjExamples/src/main/java/edu/wpi/first/wpilibj/templates/xrpcommandbased/subsystems/XRPDrivetrain.java

@@ -27,7 +27,8 @@ public class XRPDrivetrain extends SubsystemBase {
   private final Encoder m_rightEncoder = new Encoder(6, 7);
 
   // Set up the differential drive controller
-  private final DifferentialDrive m_diffDrive = new DifferentialDrive(m_leftMotor, m_rightMotor);
+  private final DifferentialDrive m_diffDrive =
+      new DifferentialDrive(m_leftMotor::set, m_rightMotor::set);
 
   /** Creates a new XRPDrivetrain. */
   public XRPDrivetrain() {

wpilibjExamples/src/main/java/edu/wpi/first/wpilibj/templates/xrpeducational/XRPDrivetrain.java

@@ -26,7 +26,8 @@ public class XRPDrivetrain {
   private final Encoder m_rightEncoder = new Encoder(6, 7);
 
   // Set up the differential drive controller
-  private final DifferentialDrive m_diffDrive = new DifferentialDrive(m_leftMotor, m_rightMotor);
+  private final DifferentialDrive m_diffDrive =
+      new DifferentialDrive(m_leftMotor::set, m_rightMotor::set);
 
   /** Creates a new XRPDrivetrain. */
   public XRPDrivetrain() {

wpilibjExamples/src/main/java/edu/wpi/first/wpilibj/templates/xrptimed/XRPDrivetrain.java

@@ -26,7 +26,8 @@ public class XRPDrivetrain {
   private final Encoder m_rightEncoder = new Encoder(6, 7);
 
   // Set up the differential drive controller
-  private final DifferentialDrive m_diffDrive = new DifferentialDrive(m_leftMotor, m_rightMotor);
+  private final DifferentialDrive m_diffDrive =
+      new DifferentialDrive(m_leftMotor::set, m_rightMotor::set);
 
   /** Creates a new XRPDrivetrain. */
   public XRPDrivetrain() {