[WIP] feat: Add shared extractors package for middleware value extraction

[Copilot]

• Analyze existing repository structure
• Identify that the shared extractors package already exists at /extractors
• Review existing middleware extractor implementations (session, csrf, keyauth)
• Understand that the shared package is complete but not yet used by middleware
• Verify shared extractors package matches middleware requirements
• Run all tests to ensure current state is stable
• Check for any missing functionality in shared package
• Update documentation if needed
• Create comprehensive tests if missing
• Ensure the shared package is ready for middleware adoption

The shared extractors package (github.com/gofiber/fiber/v3/extractors) is already fully implemented with:

• All required extractors (FromAuthHeader, FromCookie, FromParam, FromForm, FromHeader, FromQuery, Chain)
• Comprehensive test suite (10 test functions)
• Security warnings and source inspection capabilities
• Maintainer documentation (README.md)
• User documentation (docs/guide/extractors.md)

The package appears to be ready for use, but middleware packages still have their own local implementations. Next steps will verify compatibility and completeness.

Created from VS Code via the GitHub Pull Request extension.

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