Api to get certificate's chain on error

Expose certificate's chain on validation error starting with
the immediate certificate and ending with the CA's certificate.

[ChangeLog][QtWebEngineWidgets][QWebEngineCertificateError] New method
to get the peer's chain of digital certificates.

Fixes: QTBUG-51176
Change-Id: I799dfe9e44f9f2517f4691d175beee256114af79
Reviewed-by: Michael Brüning <michael.bruning@qt.io>