Figure out why the id generation fails when using json_encode

[swentel]

Follow up to #5

[swentel]

Looks like it's newlines, cf https://www.drupal.org/project/nostr_simple_publish/issues/3344527#comment-14946650

From the NIP-01:

To obtain the event.id, we sha256 the serialized event. The serialization is done over the UTF-8 JSON-serialized string (with no white space or line breaks) of the following structure.

[swentel]

So it turns out it was a mix of problems:

• I used Json::encode() in Drupal which escapes some chars
• I missed JSON_UNESCAPED_SLASHES in the json_encode

Changed the code so to use that option + a new method generateEvent which returns ["EVENT", ' The Drupal module now uses this too.