The idea is to have QSslKey(Private) backend-neutral and hide
all library-specific code inside plugins.
Fixes: QTBUG-90953
Task-number: QTBUG-65922
Change-Id: I2eeee3b2b72c78c2e24f2fb914abce3caa913be8
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
(cherry picked from commit 0352cf8e1bf57615b9faf3f6f383896444e762ac)
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>