Add DutyCycleEncoder channel constructor (#2158)

Avoids extra boilerplate at user level.

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.
    *