Scanning: eSCL support

[michaelrsweet]

Add eSCL support to PAPPL; no publicly available specification exists, but here are some online resources:

• https://testcluster.blogspot.com/2014/03/scanning-from-apple-airprint-airscan.html
• https://github.com/Cubix651/escl-scanner
• https://github.com/Ordissimo/scangearmp2/issues/7
• https://github.com/alexpevzner/sane-airscan

The latter provides a SANE backend that supports scanning via eSCL and can act as a test client.

Support functions should go in a file named scanner-escl.c.

[tillkamppeter]

The eSCL specification got actually published a year ago: https://openprinting.github.io/OpenPrinting-News-May-2021/#driverless-scanning https://mopria.org/spec-download