ot::TaskletContext class

Defines a tasklet that also maintains a user context pointer. In typical `Tasklet` use, in the handler callback, the owner of the tasklet is determined using `GetOwner` method. This method works if there is a single instance of `Type` within OpenThread instance hierarchy. The `TaskletContext` is intended for cases where there may be multiple instances of the same class/type using a `Tasklet` object. `TaskletContext` will store a context `void *` information.