cloudapi: support ostree options

Move OSTree option handling outside of the weldr API to make it usable
by other packages. New subpackage at internal/ostree.

Add support for ostree options ("Ref" and "URL") in the Cloud API.
Validate OSTree options and resolve the parent reference the same way as
in the Weldr API.

Unlike the Weldr API, the Cloud API doesn't support specifying the
Parent reference directly.

The exports list is included in the job information on the queue.

internal/cloudapi/openapi.yml

@@ -187,6 +187,8 @@ components:
           type: array
           items:
             $ref: '#/components/schemas/Repository'
+        ostree:
+          $ref: '#/components/schemas/OSTree'
         upload_request:
           $ref: '#/components/schemas/UploadRequest'
     Repository:
@@ -378,6 +380,14 @@ components:
           example: ['postgres']
           items:
             type: string
+    OSTree:
+      type: object
+      properties:
+        url:
+          type: string
+        ref:
+          type: string
+          example: ['rhel/8/x86_64/edge']
     Subscription:
       type: object
       required: