add project variables support

[tellespallabob]

Project client provides support to retrieve/add/update/delete environment variables for a given project

[codecov-io]

Codecov Report

Merging #72 into master will decrease coverage by 0.4%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #72      +/-   ##
==========================================
- Coverage   24.07%   23.67%   -0.41%     
==========================================
  Files         115      118       +3     
  Lines        1537     1563      +26     
  Branches      111      111              
==========================================
  Hits          370      370              
- Misses       1093     1119      +26     
  Partials       74       74

Impacted Files	Coverage Δ
.../Variables/Request/UpdateProjectVariableRequest.cs	0% <0%> (ø)
.../Models/Variables/Request/CreateVariableRequest.cs	0% <0%> (ø)
...LabApiClient/Models/Variables/Response/Variable.cs	0% <0%> (ø)
src/GitLabApiClient/ProjectsClient.cs	11.9% <0%> (-1.99%)	⬇️

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[nmklotas]

Hello,

Thanks for the pr, can you check why the formatting fails in the CI?

[jetersen]

I would really hope we could rework this PR to align with #68

[jetersen]

https://github.com/nmklotas/GitLabApiClient/commit/fd028dd8e94a53b9bd3f55fa6f568531c668ebb3.patch
@tellespallabob
You might want to set up an email alias on your GitHub account 🥇
Or look into fixing your git user.email config 😅
to get proper credit for your commits 🏆
https://help.github.com/articles/adding-an-email-address-to-your-github-account/
https://help.github.com/articles/setting-your-commit-email-address-in-git
then to amend your commits: https://stackoverflow.com/a/3042512

Regarding the work in #68 feel free to merge this one, I will rebase my work.