Only fetch MAD manifesto on server join

[Alystrasz]

Previously, the verified mods manifesto was fetched on game start without checking if the verified mod feature is enabled Squirrel
-side; with this, the manifesto is only fetched when the user wants to download a mod (meaning they enabled the feature beforehand).

(must be merged with the mods PR!)

Launcher changes

• Expose FetchModsListFromAPI method to Squirrel VM
• Do not invoke FetchModsListFromAPI method on client start
• Add ModInstallState.MANIFESTO_FETCHING enum value to better track mod install progress

TODOs

• Mods counterpart PR (Only fetch MAD manifesto on server join NorthstarMods#821)

Testing

Without PRs:

1. Launch game;
2. Check the logs: they should display verified mods manifesto was fetched close to game launch;

With PRs:

1. Install both launcher and mods PRs;
2. Launch game, reach MP lobby and enable MAD (allow_mod_auto_download 1);
3. Try and connect to a server requiring verified mods (Space battle or Parkour server should be up at the time of writing);
4. Check the logs: manifesto should be fetched when trying to join the server.

Media

delayed_manifesto_fetching.webm

(if you look closely, you can see verified mods manifesto is fetched WHEN joining the server, around 0:12 in the video)

[EladNLG]

Reviewed it all, don't see anything needing changing, not tested.

[GeckoEidechse]

As far as I understand the code changes they seem correct to me ^^

[GeckoEidechse]

Confirmed working in testing together with R2Northstar/NorthstarMods#821

Enabled MAD and joined a server. Observed the delayed manifesto fetching as intended.