Validate pubkeys in bech32 or hex

From: https://www.drupal.org/project/nostr_simple_publish/issues/3344796 📌 Allow users to use the bech32 key Active

I've added support to convert the bech32 version to hex in https://github.com/swentel/nostr-php

When the user submits his pubkey, we need to validate the submitted value before it's stored.