Adds support for Schannel, an SSL backend for Windows, as an
alternative to OpenSSL.
[ChangeLog][QtNetwork][Ssl] Added support for Schannel on Desktop
Windows. To build Qt with Schannel support use '-schannel' during
configure.
Task-number: QTBUG-62637
Change-Id: Ic4fb8ed3657dab994f9f4a4ac5cbddc7001a0a46
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>