add config options for providing certificates and cas through config

[simskij]

Enhancement Proposal

Some users provide their own TLS certificates, and don't want it generated through CSRs. To support this we'd need to support the following config options in addition to the ca relation:

• ssl-ca
• ssl-cert
• ssl-key

See https://gitea.arif-ali.co.uk/canonical/cpe-deployments/src/branch/main/config/overlays/ssl.yaml for an example of how this works in OpenStack.

We should add secrets manually and then refer to them using juju config.

[mmkay]

Closing as completed by #321 .