feat: add TLS integration to the NMS

[patriciareinoso]

Description

This PR adds the TLS Certificates Integration to the NMS charm.
This integration is mandatory and the charm does not start the workload without it.
If the integration does not exist or it is removed the charm goes to blocked status.

Certificates and private key are stored at /support/TLS in certs mount.
The path to the certificate and private key are added to the webui config file.

The path to the CA certificate is needed to make requests to the webui from the charm.

Attention

Once traefik implements certificate transfer interface we'll be able to use that instead of integrating Traefik with self-signed-certificates. canonical/traefik-k8s-operator#407

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that validate the behaviour of the software
• I validated that new and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have bumped the version of the library