ServiceIds type

[rubdos]

In several places in storage backends, methods are called with both an Aci and Pni, where both are optional, but one of them needs to be set. Example:

    #[tracing::instrument(skip(self))]
    pub fn fetch_group_v2_pending_member(
        &self,
        id: &str,
        aci: Option<Aci>,
        pni: Option<Pni>,
    ) -> Option<orm::GroupV2PendingMember> {
        if aci.is_none() && pni.is_none() {

We probably want to provide a type ServiceIds (better name suggestion is welcome) like this:

enum ServiceIds {
    Aci(Aci),
    Pni(Pni),
    AciAndPni(Aci, Pni),
}

... together with a bunch of convenience conversion methods. Maybe this should even be a type in libsignal-protocol/core