[wpimath] Add timestamp getter to MathShared (#5091)

This makes it possible to mock the timestamp for wpimath without affecting the rest of the library.

Co-authored-by: Peter Johnson <johnson.peter@gmail.com>

wpimath/src/main/native/cpp/MathShared.cpp

@@ -5,6 +5,9 @@
 #include "wpimath/MathShared.h"
 
 #include <wpi/mutex.h>
+#include <wpi/timestamp.h>
+
+#include "units/time.h"
 
 using namespace wpi::math;
 
@@ -15,6 +18,9 @@ class DefaultMathShared : public MathShared {
   void ReportWarningV(fmt::string_view format, fmt::format_args args) override {
   }
   void ReportUsage(MathUsageId id, int count) override {}
+  units::second_t GetTimestamp() override {
+    return units::second_t{wpi::Now() * 1.0e-6};
+  }
 };
 }  // namespace