[wpilib] Add timestamp getters with configurable time base (#7378)

wpilibc/src/main/native/cpp/TimedRobot.cpp

@@ -46,6 +46,8 @@ void TimedRobot::StartCompetition() {
       break;
     }
 
+    m_loopStartTimeUs = RobotController::GetFPGATime();
+
     callback.func();
 
     // Increment the expiration time by the number of full periods it's behind
@@ -97,6 +99,10 @@ TimedRobot::~TimedRobot() {
   }
 }
 
+uint64_t TimedRobot::GetLoopStartTime() {
+  return m_loopStartTimeUs;
+}
+
 void TimedRobot::AddPeriodic(std::function<void()> callback,
                              units::second_t period, units::second_t offset) {
   m_callbacks.emplace(