Use constant-time string comparison in verifyNotification

[CasEbb]

This PR fixes a possible timing attack in Notification::verifyNotification().

The two hashes are compared using simple string comparison which is susceptible to timing attacks. It is replaced with the constant-time hash_equals() function.

[danielcivit]

Hello @CasEbb

Thanks for submitting this contribution.

Replacing the direct string comparison with hash_equals() is a great security enhancement, as you explained, we understand it prevents potential timing attacks during hash verification.

We will be processing a new release soon that will include this change.