Function required_server_signatures_to_verify_event 

pub fn required_server_signatures_to_verify_event(
    object: &CanonicalJsonObject,
    rules: &SignaturesRules,
) -> Result<BTreeSet<OwnedServerName>, VerificationError>

Get the list of servers whose signature must be checked to verify the given event.

Applies the rules for validating signatures on received events for populating the list:

• Add the server of the sender, except if it’s an invite event that results from a third-party invite.
• For room versions 1 and 2, add the server of the event_id.
• For room versions that support restricted join rules, if it’s a join event with a join_authorised_via_users_server, add the server of that user.