Add SteamKit version as default user-agent

[yaakov-h]

Resolves #618.

[codecov-io]

Codecov Report

Merging #620 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #620      +/-   ##
==========================================
+ Coverage   23.55%   23.61%   +0.06%     
==========================================
  Files          85       85              
  Lines        8709     8714       +5     
  Branches      719      719              
==========================================
+ Hits         2051     2058       +7     
+ Misses       6526     6523       -3     
- Partials      132      133       +1

Impacted Files	Coverage Δ
...mClient/Configuration/SteamConfigurationBuilder.cs	100% <100%> (ø)	⬆️
SteamKit2/SteamKit2/Steam/WebAPI/WebAPI.cs	41.34% <0%> (-0.49%)	⬇️
SteamKit2/SteamKit2/Types/SteamID.cs	79.42% <0%> (-0.37%)	⬇️
SteamKit2/SteamKit2/Util/DebugLog.cs	67.18% <0%> (+6.25%)	⬆️

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