[locator] adding OwnerLocator (#2346)

This commit adds a new class `OwnerLocator` which is used by the
callback providing types (like `Timer` or `Tasklet`) to remember the
owner of the object. This change help simplify the handling of
callbacks from such objects. The object itself can be used to get
to its owner using `GetOwner<Type>()` method.  If support for multiple
OpenThread instances is enabled, an `OwnerLocator`  object maintains a
pointer to the owner. But for the single OpenThread instance scenario,
the owner is derived from the single `ot::Instance` object.

src/cli/cli.cpp

@@ -3536,13 +3536,13 @@ void Interpreter::SetUserCommands(const otCliCommand *aCommands, uint8_t aLength
     mUserCommandsLength = aLength;
 }
 
-Interpreter &Interpreter::GetOwner(const Context &aContext)
+Interpreter &Interpreter::GetOwner(OwnerLocator &aOwnerLocator)
 {
 #if OPENTHREAD_ENABLE_MULTIPLE_INSTANCES
-    Interpreter &interpreter = *static_cast<Interpreter *>(aContext.GetContext());
+    Interpreter &interpreter = (aOwnerLocator.GetOwner<Interpreter>());
 #else
     Interpreter &interpreter = Uart::sUartServer->GetInterpreter();
-    OT_UNUSED_VARIABLE(aContext);
+    OT_UNUSED_VARIABLE(aOwnerLocator);
 #endif
     return interpreter;
 }