MINOR: sample: add ub64dec and ub64enc converters

MINOR: sample: add ub64dec and ub64enc converters

ub64dec and ub64enc are the base64url equivalent of b64dec and base64
converters. base64url encoding is the "URL and Filename Safe Alphabet"
variant of base64 encoding. It is also used in in JWT (JSON Web Token)
standard.
RFC1421 mention in base64.c file is deprecated so it was replaced with
RFC4648 to which existing converters, base64/b64dec, still apply.

Example:
  HAProxy:
    http-request return content-type text/plain lf-string %[req.hdr(Authorization),word(2,.),ub64dec]
  Client:
    Token=eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJ1c2VyIjoiZm9vIiwia2V5IjoiY2hhZTZBaFhhaTZlIn0.5VsVj7mdxVvo1wP5c0dVHnr-S_khnIdFkThqvwukmdg
    $ curl -H "Authorization: Bearer ${TOKEN}" http://haproxy.local
    {"user":"foo","key":"chae6AhXai6e"}