Added capabilities endpoint

[WeTurkstra]

Description

Added the capabilities endpoint.

Motivation and context

Capabilities are currently not available on the client. I need the capabilities to see if a shipment can be created for a set of parameters.

How has this been tested?

The endpoint is a simple GET endpoint. There for a unit test is added to test if the related objects are filled correctly.

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)
• [x ] 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.

Please, please, please, don't send your pull request until all of the boxes are ticked.

• [x] I have read the CONTRIBUTING document.
• [x] My pull request addresses exactly one patch/feature.
• [x] I have created a branch for this patch/feature.
• [x] Each individual commit in the pull request is meaningful.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.
• [x] If my change requires a change to the documentation, I have updated it accordingly.
• [x] My code follows the code style of this project.

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[WeTurkstra]

Hi, could you please approve this PR?