[Feature] Add combinator option for multimap selector rules

[Feature] Add combinator option for multimap selector rules

This change adds support for structured data output from selectors in
multimap rules. Previously, selectors always produced concatenated
strings which made it impossible to send structured JSON data to
external map services.

New 'combinator' option for selector-type multimap rules:
- 'string' (default): concatenate results with delimiter (existing behavior)
- 'array': flatten all results into a flat array
- 'object': convert pairs of selectors into key-value object

Example configuration for external JSON API:
  multimap {
    MY_EXTERNAL_CHECK {
      type = "selector";
      selector = "id('from');from('smtp'):addr;id('ip');ip";
      combinator = "object";  # produces {"from": "...", "ip": "..."}
      map = {
        external = true;
        backend = "http://api.example.com/check";
        method = "body";
        encode = "json";
      };
    }
  }

Changes:
- lua_selectors: Added combinator registry and helper functions
  - get_combinator(name): returns combinator function by name
  - list_combinators(): returns available combinator names
  - create_selector_closure_with_combinator(): creates closure with named combinator
- multimap: Added 'combinator' option support for selector and redis+selector maps