[takelet] add template TaskeltIn to simplify handler functions (#8064)

This class adds a template sub-class of `Tasklet` which allows
us to directly specify an `Owner` of tasklet along with handler
callback as a member method of `Owner`. This helps simplify the
use of `Tasklet` in core module.

src/core/thread/dua_manager.cpp

@@ -54,7 +54,7 @@ RegisterLogModule("DuaManager");
 
 DuaManager::DuaManager(Instance &aInstance)
     : InstanceLocator(aInstance)
-    , mRegistrationTask(aInstance, DuaManager::HandleRegistrationTask)
+    , mRegistrationTask(aInstance)
     , mDuaNotification(UriPath::kDuaRegistrationNotify, &DuaManager::HandleDuaNotification, this)
     , mIsDuaPending(false)
 #if OPENTHREAD_CONFIG_DUA_ENABLE
@@ -402,11 +402,6 @@ void DuaManager::HandleTimeTick(void)
     UpdateTimeTickerRegistration();
 }
 
-void DuaManager::HandleRegistrationTask(Tasklet &aTasklet)
-{
-    aTasklet.Get<DuaManager>().PerformNextRegistration();
-}
-
 void DuaManager::UpdateTimeTickerRegistration(void)
 {
     if (mDelay.mValue == 0)