New POST option for JSON datasources

To give users more flexibility when connecting to APIs, JSON datasources now support both GET and POST methods. This makes it possible to fetch data from APIs that require request bodies or POST parameters.
- A new dropdown is available before the JSON link field with options: GET (default) and POST.
- When GET is selected, the Body field is hidden and the request is sent as a standard GET request.
- When POST is selected, a Body (raw) text area appears for entering JSON or text content.
- Custom request headers continue to work for both methods.
- If a POST request is sent with an empty body, it remains valid.
This enhancement expands integration possibilities for templates using JSON datasources, as more and more APIs are using POST.