[Feature] Add rspamd_util.decode_html_entities and improve obfuscated URL detection

[Feature] Add rspamd_util.decode_html_entities and improve obfuscated URL detection

- Add Lua binding for HTML entity decoding (rspamd_util.decode_html_entities)
  wrapping rspamd_html_decode_entitles_inplace C function
- Switch obfuscated URL detection from regexp module to rspamd_trie
  for Hyperscan-accelerated multi-pattern matching
- Fix URL flag passing (use url.create with flags table instead of add_flag)
- Fix inject_url usage (doesn't return value)
- Add functional tests for obfuscated URL detection