Remove art4/json-api-client dependency

[JaZo]

Description

I removed the art4/json-api-client dependency and implemented the validation in this package.

Motivation and context

The dependency had some major changes and it was a lot of work to update the package. We can easily validate and parse the json ourselves, so I just removed the dependency. Besides, this dependency gave licensing issues because it is licensed under GPL. This closes #56.

N.B. I consider this a bugfix as the public API isn't changed.

How has this been tested?

Tested with existing and 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.

[JaZo]

Scrutinizer says the overall code quality decreased because of all the conditions, but I don't see how I can change that as they are all justified.