Server::Service trait has no mechanism to obtain request's unit ID in call implementation

[fdlg]

For an RTU server, it's important to know a request's unit ID when the devices are on a bus. For example, on an RS-485 bus, all devices will receive all requests but only the intended receipient should send a reply.

For a TCP server, knowing the request's unit ID allows implementing logical devices or the implementation of a gateway that forwards a request to an RTU device.

The rtu and tcp server implementation already have the unit id readily available in the Header struct that they get in a frame:

tokio-modbus/src/server/rtu.rs

Lines 89 to 90 in 996511c

	let hdr = request.hdr;
	let response = service.call(request.pdu.0).await.map_err(Into::into)?;

tokio-modbus/src/server/tcp.rs

Lines 147 to 148 in 996511c

	let hdr = request.hdr;
	let response = service.call(request.pdu.0).await.map_err(Into::into)?;

The first potential solution that comes to mind is modifying Server::Service::call's signature to this:

fn call(&self, slave: Slave, req: Self::Request) -> Self::Future;

[fdlg]

I prototyped that change in the following PR in case it's something we want to do:
#97

[uklotzde]

Fixed by #141