Mod download UI integration

[Alystrasz]

UI integration of the mod downloading feature merged in #545

When downloading a mod, this will display download progress, then extraction progress onscreen. Once mod has been downloaded, mods will be reloaded, and server join will pursue.

NB: NSReloadMods currently doesn't support all mods (weapon skins for instance); some maps are supported.

Mods PR: R2Northstar/NorthstarMods#761

Test instructions

• Install launcher PR
• Install mods PR
• Enable mod downloading in Controls menu (this is now done by switching the allow_mod_auto_download convar to 1)
• Join a server ("Space battle" test server should up for you to try this feature, if not ping me please)

Media

Here's a demo of end-to-end custom map download:

autodl-frostbite-demo.mp4

Changes

• Expose download function to squirrel VM;
• modState variable is updated regarding mod download state (downloading, extracting etc);
• NSGetModInstallState function returns mod download state as a ModInstallState struct (defined in squirrel);
• ModFetchingProgressCallback function is used to get curl download progress information;
• GetModArchiveSize function gives information for extraction progress stats;
• FetchModSync isn't run in a thread anymore (since the whole downloading process is already run in a separate thread).

[GeckoEidechse]

Confirmed working in testing together with R2Northstar/NorthstarMods#761

I tested by attempting to join the Space battle server. First with the convar still set to 0 and subsequently being unable to join. Then setting the convar to 1 and rejoining triggered the auto-download and I was able to successfully join the server and play the custom map.

[GeckoEidechse]

@ASpoonPlaysGames @Jan200101 given that the two of you reviewed #545 you wanna do code review on this one as well?

Testing was already done by me so only code review is left before we can merge this ^^

[ASpoonPlaysGames]

Code looks good as far as I'm concerned