ARROW-13975: [C++] Implement decimal round

cpp/src/arrow/compute/api_scalar.cc

@@ -283,8 +283,11 @@ RoundOptions::RoundOptions(int64_t ndigits, RoundMode round_mode)
 constexpr char RoundOptions::kTypeName[];
 
 RoundToMultipleOptions::RoundToMultipleOptions(double multiple, RoundMode round_mode)
+    : RoundToMultipleOptions(std::make_shared<DoubleScalar>(multiple), round_mode) {}
+RoundToMultipleOptions::RoundToMultipleOptions(std::shared_ptr<Scalar> multiple,
+                                               RoundMode round_mode)
     : FunctionOptions(internal::kRoundToMultipleOptionsType),
-      multiple(multiple),
+      multiple(std::move(multiple)),
       round_mode(round_mode) {}
 constexpr char RoundToMultipleOptions::kTypeName[];
 

cpp/src/arrow/compute/api_scalar.h

@@ -93,10 +93,17 @@ class ARROW_EXPORT RoundToMultipleOptions : public FunctionOptions {
  public:
   explicit RoundToMultipleOptions(double multiple = 1.0,
                                   RoundMode round_mode = RoundMode::HALF_TO_EVEN);
+  explicit RoundToMultipleOptions(std::shared_ptr<Scalar> multiple,
+                                  RoundMode round_mode = RoundMode::HALF_TO_EVEN);
   constexpr static char const kTypeName[] = "RoundToMultipleOptions";
   static RoundToMultipleOptions Defaults() { return RoundToMultipleOptions(); }
-  /// Rounding scale (multiple to round to)
-  double multiple;
+  /// Rounding scale (multiple to round to).
+  ///
+  /// Should be a scalar of a type compatible with the argument to be rounded.
+  /// For example, rounding a decimal value means a decimal multiple is
+  /// required. Rounding a floating point or integer value means a floating
+  /// point scalar is required.
+  std::shared_ptr<Scalar> multiple;
   /// Rounding and tie-breaking mode
   RoundMode round_mode;
 };