Add public modifier to 3 classes to enable linking

[shalzuth]

Setting these 3 classes as public allows another C# app to link to the exe and use the it as a reference to download files.
Enables my feature request - #557

[xPaw]

Can you show an example that would use this?

[shalzuth]

I use it for a Helldivers 2 site I run. I download the content files on the server I host to have auto-updates enabled (and it runs on Linux so it's a bit tricky). I can't run the command line as-is, so I recreate the entry point of DepotDownloader effectively.
I use mostly the same code from https://github.com/SteamRE/DepotDownloader/blob/master/DepotDownloader/Program.cs
https://gist.github.com/shalzuth/440670dbd2b86db1887a7e0fda3c8acf is the code I use. I also do some funny things to make it so I can re-use it within the same process multiple times and not fail on init.

[xPaw]

I mean even if we merge this, it still won't be on nuget or be published nupkg, so you'll have to pull in the source/gitmodule this repo.

[shalzuth]

I'd consider this the first of multiple PRs to enable the functionality, to eventually be on nuget.

[yaakov-h]

Previous discussion about that was here: #146 (comment)

[xPaw]

Yeah I agree, I don't think the way this app is implemented is a good idea to expose as a public api.