[Feature] Add pluggable hyperscan cache storage infrastructure

[Feature] Add pluggable hyperscan cache storage infrastructure

This commit adds infrastructure for pluggable hyperscan cache storage
backends and FD-based shared memory distribution:

- Add platform ID function (rspamd_hyperscan_get_platform_id) for
  platform-aware cache keys
- Create lua_hs_cache.lua with file, Redis, and HTTP backends
- Add FD-based loading APIs (rspamd_hyperscan_from_fd,
  rspamd_hyperscan_create_shared_unser)
- Add fd_size field to control messages for FD passing
- Update worker to handle attached FDs in hyperscan notifications
- Add cache_backend configuration option to hs_helper