Make DepotManifest constructor public

[xPaw]

Fixes #840

I successfuly used it in https://github.com/xPaw/SteamDepotFileVerifier by using reflection.

[codecov]

Codecov Report

Merging #841 into master will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #841      +/-   ##
==========================================
- Coverage   22.75%   22.75%   -0.01%     
==========================================
  Files          95       95              
  Lines        9343     9344       +1     
  Branches      780      780              
==========================================
  Hits         2126     2126              
- Misses       7083     7084       +1     
  Partials      134      134              

Impacted Files	Coverage Δ
SteamKit2/SteamKit2/Types/DepotManifest.cs	0.00% <0.00%> (ø)

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[yaakov-h]

You can probably simplify your “dumb” hack with Activator.CreateInstance.

Id actually rather make this a static method. I’m pretty firmly in the “constructors should do no work” camp and boy is this one doing a lot of work.