Add optional "prototype" property to applications and expose it in generated JSON

[wlach]

While working on the Glean Dictionary, we found that it might be helpful to have a "prototype" property to distinguish "applications" like GleanJs which are in development (and where we might expect to find e.g. non-existent data review).

@singhvaishnavi would you like to give this a try as a first bug? It's python/docker (not svelte). You can base work here off of the approach in #220. If you take it on, please add some tests to make sure things as working as expected while you're here.

@brizental has volunteered to mentor/review this.

[brizental]

Feel free to reach out to me on Matrix if necessary. My username there is also brizental.

[singhvaishnavi]

Yes, I would like to work on this.