new XCertificateCreator interface which can be used to create a
certificate and a associated private key, both in DER format

Change-Id: I24caf053f1f2b7357a836047b88b4ce418470eac
Reviewed-on: https://gerrit.libreoffice.org/61912
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>
(cherry picked from commit bdde44d5)