[bug] REST API does not allow specifying upgrade_options for upgrade operations

[pandafy]

Describe the bug
The REST API for openwisp-firmware-upgrader does not allow specifying upgrade_options when initiating firmware upgrade operations. While the backend and Django admin UI support setting and validating upgrade_options (added in #226), the REST API endpoints and serializers for UpgradeOperation and batch upgrades do not expose or accept this field. This limits automation and integration since users cannot programmatically supply upgrade parameters via the API.

Expected behavior
The REST API should:

• Accept an upgrade_options (JSON) field for upgrade/batch upgrade endpoints, validating it against the appropriate schema.
• Expose upgrade_options in relevant serializers so its value can be retrieved from API responses for upgrade operations.
• Achieve feature parity with the Django admin interface for specifying firmware upgrade parameters.