add new landing page integration

[simskij]

This PR introduces a new relation interface called catalogue for connecting Prometheus to a landing page. Over this relation, Prometheus provides an application entry, which then will be used by the landing page to display where users may go.

Context

See https://github.com/canonical/catalogue-k8s-operator/ for the other end of this relation.

Testing Instructions

• Build a charm file from this PR branch
• Deploy the build output on Juju
• Deploy catalogue-k8s from edge
• Relate the two charms together
• Visit the landing page on port 80

Release Notes

Implement the catalogue interface to allow for integration with catalogue-k8s.