Add DutyCycleEncoder channel constructor (#2158)

Avoids extra boilerplate at user level.

wpilibc/src/main/native/cpp/DutyCycleEncoder.cpp

@@ -8,6 +8,7 @@
 #include "frc/DutyCycleEncoder.h"
 
 #include "frc/Counter.h"
+#include "frc/DigitalInput.h"
 #include "frc/DigitalSource.h"
 #include "frc/DriverStation.h"
 #include "frc/DutyCycle.h"
@@ -15,6 +16,12 @@
 
 using namespace frc;
 
+DutyCycleEncoder::DutyCycleEncoder(int channel)
+    : m_dutyCycle{std::make_shared<DutyCycle>(
+          std::make_shared<DigitalInput>(channel))},
+      m_analogTrigger{m_dutyCycle.get()},
+      m_counter{} {}
+
 DutyCycleEncoder::DutyCycleEncoder(DutyCycle& dutyCycle)
     : m_dutyCycle{&dutyCycle, NullDeleter<DutyCycle>{}},
       m_analogTrigger{m_dutyCycle.get()},

wpilibc/src/main/native/include/frc/DutyCycleEncoder.h

@@ -32,6 +32,13 @@ class DutyCycleEncoder : public ErrorBase,
                          public Sendable,
                          public SendableHelper<DutyCycleEncoder> {
  public:
+  /**
+   * Construct a new DutyCycleEncoder on a specific channel.
+   *
+   * @param channel the channel to attach to
+   */
+  explicit DutyCycleEncoder(int channel);
+
   /**
    * Construct a new DutyCycleEncoder attached to an existing DutyCycle object.
    *