Implement "certificates" interface to enable automatic CA provisioning

Hello,

We need to implement "certificates" interface for openstack-service-checks charm to enable automatic CA provisioning via "juju add-relation osc vault" or by configuring relations in bundles.

Right now there is a workaround how to do this:
```
juju run-action \
    --wait vault/0 get-root-ca --format json \
    | jq -r '."unit-vault-0".results.output' \
    | base64 -w 0 \
    | xargs -I {} juju config openstack-service-checks trusted_ssl_ca={}
```

But it is not so obvious so user faces bugs like that:
https://bugs.launchpad.net/charm-openstack-service-checks/+bug/1926168

Best Regards,
Alex.