[Feature] Add composites statistics and config options

[Feature] Add composites statistics and config options

- Add config options: composites_inverted_index (default: true),
  composites_stats_always (default: false for 1/256 sampling)
- Add EMA-based timing statistics using rspamd_counter_data
- Export stats via /stat endpoint in "composites" section:
  checked_slow, checked_fast, matched counts and timing with mean/stddev
- Allow toggling inverted index for performance comparison