fix: Resolve typecheck errors with extension field types
- Changed parseExtensions to use any types instead of generic constraints
- Removed SpecificationExtensions from interfaces with index signatures (CallbackObject, PathsObject, ResponsesObject)
- Changed PathsObject index signature from /${string} to string for compatibility
- These changes fix TypeScript errors while preserving extension field support
Co-authored-by: mrlubos <12529395+mrlubos@users.noreply.github.com>