Integrate jupyter-ui with ambient mesh charms

### Context

We'll need to ensure the `jupyter-ui` charm can be integrated with the ambient mesh.

### What needs to get done

1. Handle the relation with `istio-beacon`
2. Integrate the charm with the Gateway charm

### Definition of Done

1. The charm can work with both sidecar and mesh architecture
2. If related to beacon and gateway, then only the gateway can talk to the app