Add plugins.json format version field #400
Description
The plugins.json file syntax can be versioned, which allows renaming properties, for example (see this issue and this document.
The version is present in the URL, e.g. https://list-v1--netlify-plugins.netlify.app/plugins.json. However, it would be good to make it available in the plugins.json HTTP response as well.
At the moment, that file is a top-level array. Introducing that new field would require making it a top-level object instead. This would be a breaking change of the plugins.json syntax (see this) which would require updating every consumer.