Improve CORS functionality implementation

[marevol]

This commit improves the CORS (Cross-Origin Resource Sharing) implementation in the org.codelibs.fess.cors package with the following enhancements:

1. Bug Fixes:

   • Fixed typo in CorsHandlerFactory: renamed 'handerMap' to 'handlerMap'
   • Changed OPTIONS preflight response from SC_ACCEPTED (202) to SC_OK (200) for better compliance with CORS standard

2. Thread Safety:

   • Replaced HashMap with ConcurrentHashMap in CorsHandlerFactory
   • Added thread-safety documentation

3. New Features:

   • Added Access-Control-Expose-Headers support to expose custom headers
   • Added Vary: Origin header for proper caching behavior
   • Added Access-Control-Allow-Private-Network support for private network access requests (Chrome feature)

4. Configuration:

   • Added new config property: api.cors.expose.headers
   • Added corresponding method in FessConfig interface
   • Updated default values and documentation

5. Code Quality:

   • Added inline comments for better code readability
   • Added new header constants in CorsHandler base class
   • Improved JavaDoc documentation

6. Tests:

   • Updated CorsHandlerFactoryTest to use correct field name
   • Updated CorsFilterTest to expect SC_OK instead of SC_ACCEPTED

These improvements enhance security, performance, and standards compliance of the CORS implementation while maintaining backward compatibility.