ot::InstanceLocatorInit class

Implements a locator for an OpenThread Instance object. The `InstanceLocatorInit` is similar to `InstanceLocator` but provides a default constructor (instead of a parameterized one) and allows an inheriting class to initialize the object (set the OpenThread Instance) post constructor call using the `Init()` method. This class is intended for types that require a default constructor and cannot use a parameterized one. (e.g., `Neighbor`/`Child`/`Router` classes which are used as a C array element type in`ChildTable`/`RouterTable`). The inheriting class from `InstanceLocatorInit` should ensure that object is properly initialized after the object is created and more importantly that it is re-initialized when/if it is cleared or reset.