REST API endpoints to create a custom field and to create, edit and delete custom fields option are already available. The ones to edit and delete custom fields are still missing.
For further details please refer to the Jira Cloud REST API documentation
It would be nice to be able to deal with custom fields using Jira REST API.
At the moment, it's possible to get some info by retrieving editmeta :
But you cannot add, edit, or delete custom fields via JIRA REST API (as you can in the admin section).
None at present for Cloud