OpenAPI - ExtSources manager

Created by: xkostka2

• The goal of this task is to replace all usages of methods from extSources service with methods generated from OpenAPI.
• Some methods will probably be missing, and they need to be added to the openApi specification. This file is located in the main perun repository, inside perun-openapi module.
• If you want to test your added methods, you can edit the file ng-openapi-gen.json which declares the location of the specification file.
• Don't forget to run openapi-gen to update your locally generated methods from the latest specification.