Add WebUrl property to Commit

[ap0llo]

Issues, Merge Requests and Milestones expose a WebUrl property that contains the link to the GitLab web interface for the item.
The Commit class does not have such a property, but the server response includes it (according to the GitLab API docs)

This PR adds the WebUrl property to the Commit class as well.

The PR also adds a global.json to pin the .NET Core SDK version to 3.1.100 because the build seems to be broken on the recently released SDK 3.1.200 (dotnet/sdk#10878)

[jetersen]

@ap0llo mind reverting 9151b30 ? and merge or rebase with master.

[codecov-io]

Codecov Report

Merging #127 into master will decrease coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #127      +/-   ##
==========================================
- Coverage   71.06%   71.02%   -0.04%     
==========================================
  Files         151      151              
  Lines        2139     2140       +1     
==========================================
  Hits         1520     1520              
- Misses        619      620       +1     

Impacted Files	Coverage Δ
...GitLabApiClient/Models/Commits/Responses/Commit.cs	15.38% <0.00%> (-1.29%)	⬇️

[ap0llo]

@jetersen Sure. I removed 9151b30 and rebased onto master