[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/common/notifier.cpp

@@ -46,7 +46,7 @@ RegisterLogModule("Notifier");
 
 Notifier::Notifier(Instance &aInstance)
     : InstanceLocator(aInstance)
-    , mTask(aInstance, Notifier::EmitEvents)
+    , mTask(aInstance)
 {
     for (ExternalCallback &callback : mExternalCallbacks)
     {
@@ -118,11 +118,6 @@ void Notifier::SignalIfFirst(Event aEvent)
     }
 }
 
-void Notifier::EmitEvents(Tasklet &aTasklet)
-{
-    aTasklet.Get<Notifier>().EmitEvents();
-}
-
 void Notifier::EmitEvents(void)
 {
     Events events;