Add create and update custom fields mutations

What does this MR do and why?

This adds the GraphQL mutation and service classes to create and update custom field configurations.

This does not handle adding / updating the associated select field options or associated work item types yet. I'll do that in a separate MR.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Related to #479547 (closed)