feat: add new updateOption parameter

[jameskim0987]

Resolves #60

Changes made in this PR:

1. add new field UpdateOption in SourceStringsUploadRequest according to https://support.crowdin.com/developer/api/v2/string-based/#tag/Source-Strings/operation/api.projects.strings.uploads.post. Also added comment for the field outlining the default value and enum values
2. add validation for UpdateOption. To use this field, the field UpdateStrings must be set to true as a prerequisite
3. add testcases for invalid and valid cases

[jameskim0987]

Hi @andrii-bodnar, this PR is ready for review

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.20%. Comparing base (3ec8c35) to head (e899f78).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #63      +/-   ##
==========================================
+ Coverage   98.20%   98.20%   +0.01%     
==========================================
  Files          66       66              
  Lines        2934     2936       +2     
==========================================
+ Hits         2881     2883       +2     
  Misses         29       29              
  Partials       24       24              

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[andrii-bodnar]

@jameskim0987 thanks a lot for the contribution!