Merge SChannel/Windows backend into glib-networking
The SChannel backend can be found here: https://github.com/centricular/glib-schannel
By merging it with glib-networking, it would be easier to discover and the code duplication between the two could be reduced (the base classes at least).
The SChannel backend is feature complete from what I know except for the missing DTLS support (SChannel supports DTLS though), not sending the complete certificate chain from a server connection (https://github.com/centricular/glib-schannel/issues/3) and missing support for importing PKCS#1 private keys (PKCS#8 is supported by the SChannel API).
Main advantage of the SChannel backend over the GnuTLS one is that it's already part of Windows and you don't have to ship your own TLS stack with your application, and especially not your own CA certificates that also have to be maintained. Secondarily it allows usage of private keys stored inside the Windows key store, which can't be directly accessed via any other API and usage of that key store is mandatory in various company environments.
If there's consensus that this should happen and which changes should be done to make it acceptable, I can prepare a MR.