Engines
By default, the "standard" base64 engine is used which is specified in
RFC 4648.
Additional pre-configured base64 engines are provided these are:
"standard_no_pad"
: uses the standard engine without padding
"url_safe"
: uses a url-safe alphabet with padding
"url_safe_no_pad"
: uses a url-safe alphabet without padding
See base64 crate for more.