MINOR: sample: add a generic reverse converter

MINOR: sample: add a generic reverse converter

Some use cases benefit from reversing a string before passing it to other
converters or lookups. While reverse_dom addresses domain-specific label
reversal, a generic byte-wise string reversal remains useful on its own and can
also be combined with other converters such as concat().

A common lookup use case is turning a suffix match on the original string into
a prefix match on the reversed string. 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 reversing
before map_beg can avoid linear suffix scans for large maps.

This patch adds a new string converter named "reverse". It reverses the input
string byte by byte and returns the resulting string unchanged otherwise. It
does not apply any domain-specific semantics or character-encoding semantics.

The documentation is updated and a reg-test is added to cover the basic
conversion as well as a simple composition with concat(.).