feat: implement Gitea alerting provider integration

[appleboy]

Summary

• Add TypeGitea for the gitea alerting provider
• Introduce a new file for the gitea alerting provider implementation
• Implement the AlertProvider struct with necessary fields for gitea integration
• Add validation logic for the AlertProvider configuration
• Create tests for the AlertProvider's validation and sending functionality
• Update go.mod to include the gitea SDK as a dependency
• Modify the alerting configuration validation to recognize TypeGitea
• Add Assignees support to AlertProvider functionality

Checklist

• [X] Tested and/or added tests to validate that the changes work as intended, if applicable.
• [X] Updated documentation in README.md, if applicable.

[appleboy]

@TwiN The PR is already done. Please help to review.

[appleboy]

@TwiN All Done. Thanks for your review.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 69.23077% with 28 lines in your changes missing coverage. Please review.

Project coverage is 78.69%. Comparing base (dd435a8) to head (4886b4e).

Files	Patch %	Lines
alerting/provider/gitea/gitea.go	68.88%	26 Missing and 2 partials :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #842 +/- ## ========================================== - Coverage 78.85% 78.69% -0.17% ========================================== Files 69 70 +1 Lines 4621 4712 +91 ========================================== + Hits 3644 3708 +64 - Misses 771 797 +26 - Partials 206 207 +1 ```

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