Add Group Variables CRUD methods

[sfmskywalker]

This PR adds support for interacting with group-level variables.

I wasn't sure whether I should add the group variable types to the Variables folder or stick them into the Groups folder, so I opted for the latter. But I'm happy to move them to the Variables folder if that has preference. If so, I suggest that we rename Variable to ProjectVariable to make a clear distinction between the two types of variables, and propagate this change wherever applicable. I'm also happy to move the stuff from Variables into the Projects folder.

Either way, it will be more consistent. Just say the word.

[codecov-io]

Codecov Report

Merging #146 into master will increase coverage by 0.26%.
The diff coverage is 96.00%.

@@            Coverage Diff             @@
##           master     #146      +/-   ##
==========================================
+ Coverage   71.59%   71.85%   +0.26%     
==========================================
  Files         162      165       +3     
  Lines        2299     2324      +25     
==========================================
+ Hits         1646     1670      +24     
- Misses        653      654       +1     

Impacted Files	Coverage Δ
src/GitLabApiClient/GroupsClient.cs	54.90% <90.00%> (+3.81%)	⬆️
...dels/Groups/Requests/CreateGroupVariableRequest.cs	100.00% <100.00%> (ø)
...dels/Groups/Requests/UpdateGroupVariableRequest.cs	100.00% <100.00%> (ø)
...itLabApiClient/Models/Groups/Responses/Variable.cs	100.00% <100.00%> (ø)