Implement IPP-Scan Support in go-mfp

This proposal is about adding IPP-Scan support to go-mfp, OpenPrinting’s Go toolkit for multi-function printers and scanners maintained by Alexander Pevzner. Right now, go-mfp has solid IPP support for printing and a full eSCL implementation for scanning (WS-Scan is also being added). But there’s no IPP-Scan support yet, and that’s a gap. IPP-Scan (defined in PWG 5100.17) is the open standard for driverless scanning over IPP. It’s what Scanner Applications need to move away from the current eSCL-based approach. I’m proposing to build both a client and a server for IPP-Scan - the client for testing PAPPL scanner apps, and the server as a reference implementation that will also help with developing a future SANE backend for IPP-Scan.