fix: sdcore_config interface tests

[patriciareinoso]

Description

sdcore-config interface tests in https://github.com/canonical/charm-relation-interfaces are failing. This PR adds a fixture of the NSSF operator that will be used during sdcore-config provider tests It also adds the jinja2 dependency that was missing

https://github.com/canonical/charm-relation-interfaces/actions/runs/10444868384/job/28919962915 https://juju.is/docs/sdk/write-interface-tests

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