Add new readonly plugin configuration field

[jwm0]

I want to merge this change because it adds a new plugin settings field of readonly type.

PR intended to be tested with API branch: fix/handle-plugin-webhook-per-plugin

Screenshots

Pull Request Checklist

1. This code contains UI changes
2. All visible strings are translated with proper context including data-formatting
3. Attributes [data-test-id] are added for new elements
4. Changes are mentioned in the changelog
5. The changes are tested in different browsers and in light/dark mode

Test environment config

API_URI=https://fix-handle-plugin-webhook-per-plugin.api.saleor.rocks/graphql/