feat: Add support for string comment attachments

[MHIZokuchi]

• Add CommentAttachment and CommentAttachmentRequest models
• Update AddStringCommentRequest to support attachments parameter
• Update StringComment to include attachments in response
• Implement DeleteCommentAttachment endpoint
• Add unit test for DeleteCommentAttachment method

Closes #353

[codecov]

Codecov Report

❌ Patch coverage is 42.10526% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.48%. Comparing base (f45d418) to head (59285a5).
⚠️ Report is 175 commits behind head on main.

Files with missing lines	Patch %	Lines
...rc/Crowdin.Api/StringComments/CommentAttachment.cs	0.00%	8 Missing ⚠️
...wdin.Api/StringComments/AddStringCommentRequest.cs	0.00%	1 Missing ⚠️
...din.Api/StringComments/CommentAttachmentRequest.cs	0.00%	1 Missing ⚠️
src/Crowdin.Api/StringComments/StringComment.cs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #355       +/-   ##
===========================================
+ Coverage   50.38%   68.48%   +18.10%     
===========================================
  Files         277      448      +171     
  Lines        3688     6849     +3161     
  Branches        0      563      +563     
===========================================
+ Hits         1858     4690     +2832     
- Misses       1830     2110      +280     
- Partials        0       49       +49     

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[andrii-bodnar]

@MHIZokuchi thank you!