XX-3979 / Add callback system to E2E and add feature to send e2e message to partner on e2e close

Adds a boolean to e2e.Hanlder, DeletePartner, that allows the sending of an e2e message to a partner when deleting the partnership. It also adds a callback system similar to AuthCallbacks, but optional; a general callback for all partners is set via RegisterCallbacks, and callbacks for individual partners are managed with AddPartnerCallbacks and DeletePartnerCallbacks.

To allow DeletePartner to send an e2e message to a partner after deleting them, the internal sendE2E function was split to allow the send to be prepared without actually sending via prepareSendE2E. The public API is unchanged; this is only an internal change (currently).