QtOpcUa needs to be able to create self-signed certificates and certificate signing requests

We need a new class that wraps the SDK's UaPKI module for creating certificates.

This is essential to get security working without the need of external configuration of certicates.

Only this way security gets usable for the avarage user without admin skills.

1) Creating self-signed certificates

On startup UA application should be able to create a self-signed certificate and private key file if it does not exist.

2) Creating certificate signing requests (CSR)

When implementing GDS functionality it is necesary to generate CSRs which are sent to the GDS based CA, which then issues a CA signed certificate.

Creating a cert or a CSR is a very similar task, so we should create this in preperation to support GDS.