Remove unused auth header files without implementations (#130)

- Remove headers that were placeholders without implementations:
  - auth.h, http_client.h, jwks_client.h, jwt_validator.h
  - metadata_generator.h, scope_validator.h
- Keep working headers:
  - auth_c_api.h (used by implementation)
  - memory_cache.h (template with inline implementation)
  - auth_types.h (type definitions)
- Remove test files for non-existent implementations
- Update CMakeLists.txt and remaining tests

Author: RahulHere