Created customer entity

[gbenziv]

Closes #158

• Added customer entity

Pull Request Checklist

• I have ensured that my commit message contains the Issue ID.
• I have ensured that my commit message contains the feature/bugfix description.
• I have updated the documentation accordingly.
• I have ensured my code follows the coding guidelines.
• I have ran lint in my code locally prior to submission.
• I have built my changes in local successfully.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #176 (538da71) into master (a933054) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #176   +/-   ##
=======================================
  Coverage   94.61%   94.61%           
=======================================
  Files          37       37           
  Lines         724      724           
=======================================
  Hits          685      685           
  Misses         39       39           

Impacted Files	Coverage Δ
core/entity/employee.hpp	100.00% <ø> (ø)

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