MINOR: sample: add a reverse_dom converter

MINOR: sample: add a reverse_dom converter

In domain-based routing and policy rules, suffix matching on hostnames is
often easier to express as a prefix match on reversed labels. A dedicated
converter makes this convenient with existing fetches and matchers.

This also has a performance benefit for large maps. Prefix string matches use
the prefix-tree index (PAT_MATCH_BEG with pat_idx_tree_pfx), while end matches
use the string-list index (PAT_MATCH_END with pat_idx_list_str), so
reversed-label lookups can avoid linear suffix scans.

This patch adds "reverse_dom", a string converter that reverses domain labels,
ignores one optional trailing dot on input, and rejects empty labels. It
intentionally leaves trailing-dot handling to the caller so configurations can
choose between exact matches, subdomain-only matches, or an explicit dotted
form built with "concat(.)" for prefix lookups.

Examples:
  example.com      -> com.example
  mail.example.com -> com.example.mail

The documentation is updated and a reg-test covers the converter itself, the
explicit dotted form for "map_beg()", and the subdomain-only "-m beg" case.