Added support for converting multiple JWKs

[musa11971]

This PR introduces a new convenience method that converts an array of JWKs to PEM, called multipleToPem.

Here's a quick look at how it works:

use CoderCat\JWKToPEM\JWKConverter;

// Retrieve Apple's public keys in JWK format
$appleKeys = json_decode(file_get_contents('https://appleid.apple.com/auth/keys'), true);

// Convert the keys to PEM
$converter = new JWKConverter();
$pems = $converter->multipleToPem($appleKeys['keys']);

// $pems is now an array of (string) PEMs