fix(IoT): Adding completion callbacks for registerWithShadow and unregisterFromShadow methods

[ruisebas]

Issue #, if available:

• AWSIoTDataManager.getShadow() timeout [race condition with AWSIoTDataManager.register()?] #1159

Description of changes:

This PR adds the following methods to AWSIoTDataManager:

• registerWithShadow:options:eventCallback:completionCallback:
• unregisterFromShadow:completionCallback:

They are basically just overloading the existing ones with the addition of the completionCallback argument, which is invoked when the subscription to all topics is done.

The reasoning behind this is to provide a way for consumers to know when the subscription is actually done and they can get the shadow (see the referenced issue for more information).

Check points:

• Added new tests to cover change, if needed
• All unit tests pass
• All integration tests pass
• Updated CHANGELOG.md
• Documentation update for the change if required
• PR title conforms to conventional commit style

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[ruisebas]

Having the nullable annotation in the interface method resulted in weird stuff when the method was translated to Swift and autocompletion was attempted.

While not an error itself, I rather just do this to avoid confusing consumers, i.e. having the completionCallback declared as nonnull and having an implementation-only method that can take nil.

The same applies for the unregisterFromShadow method