Explicit type hint and magic methods for id

[JaZo]

Description

I have added explicit (optional string) type hints to the setId and getId methods. This requires a change to the interface and can be breaking if you implement the interface yourself. Besides that, I have made the id accessible using magic methods.

Motivation and context

According to the specification (https://jsonapi.org/format/#document-resource-object-identification), the id must always be a string when present. Making the id accessible via magic methods allows you to set/get the id just like any other attribute. This is useful for example if you want to pluck all ids of a collection of items.

How has this been tested?

Tested with new unit tests.

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

Go over all the following points, and put an x in all the boxes that apply.

• I have read the CONTRIBUTING document.
• My pull request addresses exactly one patch/feature.
• I have created a branch for this patch/feature.
• Each individual commit in the pull request is meaningful.
• I have added tests to cover my changes.
• If my change requires a change to the documentation, I have updated it accordingly.