Add support for base64url encoding and decoding

 Add support for base64url encoding and decoding
 
 This adds support for base64url encoding and decoding, a base64
 variant which is safe to use in filenames and URLs.  base64url
 replaces '+' in the base64 alphabet with '-' and '/' with '_',
 thus making it safe for URL addresses and file systems.
 
 Support for base64url was originally suggested by Przemysław Sztoch.
 
 Author: Florents Tselai <florents.tselai@gmail.com>
 Reviewed-by: Aleksander Alekseev <aleksander@timescale.com>
 Reviewed-by: David E. Wheeler <david@justatheory.com>
 Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
 Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
 Reviewed-by: Chao Li (Evan) <li.evan.chao@gmail.com>
 Discussion: https://postgr.es/m/70f2b6a8-486a-4fdb-a951-84cef35e22ab@sztoch.pl