Add a spamchecker method to allow or deny 3pid invites (#10894)

This is in the context of creating new module callbacks that modules in https://github.com/matrix-org/synapse-dinsic can use, in an effort to reconcile the spam checker API in synapse-dinsic with the one in mainline.

Note that a module callback already exists for 3pid invites (https://matrix-org.github.io/synapse/develop/modules/third_party_rules_callbacks.html#check_threepid_can_be_invited) but it doesn't check whether the sender of the invite is allowed to send it.