base64 url safe encoding

[Gashmob]

Is your feature request related to a problem? Please describe.
For the implementation of WebAuthn in a web app, I need to send a key id in the uri. So I use base64 encoding for that. But following the rfc the encoding uses + and / characters which are not really safe for an uri. Section 5 of rfc describe an url version of encoding using - and _ characters. Is it possible to implement it inside psl?

Describe the solution you'd like
It will be nice to have functions like this:

function encodeUrl(string $binary): string;

function decodeUrl(string $base64): string;

Describe alternatives you've considered
Currently, as a workaround I'm doing that:

strtr(encode($key_id), '+/', '-_');

[azjezz]

I'm okay with adding two new functions to Psl\Encoding\Base64, or a new Psl\Encoding\Base64\Url sub-component.

a PR would be welcome.

use Psl;
use Psl\Encoding\Base64;

$data = '...';

$encoded = Base64\url_encode($data);
$decoded = Base64\url_decode($encoded);

Psl\invariant($data === $decoded, 'expected $decoded to be the same as $data');

or

use Psl;
use Psl\Encoding\Base64\Url;

$data = '...';

$encoded = Url\encode($data);
$decoded = Url\decode($encoded);

Psl\invariant($data === $decoded, 'expected $decoded to be the same as $data');