Add support for device masquerading (MSC4326)

[rsb-tbg]

The shortcoming you see
Synapse v1.141.0+ now has stable support for device masquerading. I'd like to submit a PR to add similar support for ruma-client.

The improvement
My initial thought would be to modify send_request_as from:

    pub async fn send_request_as<R: OutgoingRequest>(
        &self,
        user_id: &UserId,
        request: R,
    ) -> ResponseResult<C, R> {
        self.send_customized_request(request, add_user_id_to_query::<C, R>(user_id)).await
    }

to:

    pub async fn send_request_as<R: OutgoingRequest>(
        &self,
        user_id: Option<&UserId>,
        device_id: Option<&DeviceId>,
        request: R,
    ) -> ResponseResult<C, R> {
        self.send_customized_request(request, add_ids_to_query::<C, R>(user_id, device_id)).await
    }

I would also similarly change the send_matrix_request_as signature.

Is that acceptable? Or should I avoid breaking changes by creating independent functions for user and/or device masquearding?

[zecakeh]

Device masquerading was added to ruma in ruma/ruma#2250. I think that what we want is to upgrade Ruma and reuse AppserviceUserIdentity.

Ideally we would use OutgoingRequestAppserviceExt::try_into_http_request_with_identity() to avoid duplicating the logic of adding the identity to a request in ruma-client, but given our API that's not ideal. In my opinion, the best solution would be to expose a method on AppserviceUserIdentity to add the identity to a request, that we use in both places.

[rsb-tbg]

I'm tracking the ruma/ruma#2250 now. Thanks for pointing that out. I started on a draft PR to use OutgoingRequestAppserviceExt::try_into_http_request_with_identity() so I'll just push that in case you care to check it out but can otherwise work on the AppserviceUserIdentity method if the goal is to avoid breaking changes.

[rsb-tbg]

Implemented in #18