Add a toggle to show/hide third party apps

[robog-two]

Fixes #2113 ... sort of

#2113 was about respecting flatpak's dont-enumerate setting, but this is another way of choosing to only see first-party apps from the elementary team, and it uses a switch in the settings menu.

[danirabbit]

I think instead of doing this it would be more flexible to have a way to manage remotes so you could choose from a list which remotes to show apps from.

also, to be clear, the majority of the apps in ApCenter are third party. Only the apps that are published with elementary as the developer are 1st party :)

[robog-two]

While that is a valid point I think this is a fine way of going about it for now --- managing repositories is sort of difficult and would require much more design, as well as actually respecting flatpak's settings, which AppCenter doesn't even have a means to check as of now. (This is why I added the switch instead of reading no-enumerate) Also, I think it would be possible/better to fork the tool from Pop!_OS, I believe it's called repoman, that does excatly this and is already integrated with their AppCenter fork (Pop!_Shop)