#76: Draft implementation

[skapral]

Closes #76

Root cause of the issue

N/A

Description of the changes

Added support for X-Gitlab-Token. In case of mismatch, event is rejected.

Checklist

• No classes in this PR were explicitly annotated with @Atom or @NotAtom annotation,
  or the reason for such intention is provided in PR description.

[codecov]

Codecov Report

Merging #82 into master will increase coverage by 0.14%.
The diff coverage is 73.33%.

@@             Coverage Diff              @@
##             master      #82      +/-   ##
============================================
+ Coverage     66.24%   66.39%   +0.14%     
- Complexity       93       96       +3     
============================================
  Files            53       55       +2     
  Lines           477      491      +14     
  Branches         17       18       +1     
============================================
+ Hits            316      326      +10     
- Misses          156      159       +3     
- Partials          5        6       +1

Impacted Files	Coverage Δ	Complexity Δ
...l/puzzler/gitlab/config/Cp_GITLAB_HOOK_SECRET.java	0% <0%> (ø)	0 <0> (?)
...ub/operation/OpValidatingGithubEventSignature.java	90.9% <100%> (ø)	4 <1> (?)
...gitlab/operation/OpValidatingGitlabEventToken.java	83.33% <83.33%> (ø)	3 <3> (?)

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