feed.json supports one line summary and long description

[guoyunhe]

I am developing a AppImageHub backend for Discover, the KDE Plasma app store.

Discover shows descriptions in two format:

1. one line short summary (plain text)
2. long description in multiple paragraphs (html)

AppStream supports summary and description fields. However, feed.json doesn't expose summary field.

It would be great if feed.json can support both summary and description.