Implement template processing for the index.html file to solve the UI serving in charms

[BarcoMasile]

Description

When the Admin UI is deployed on Juju it will be served behind Traefik, with a specific context path.
For example, if traefik url is https://traefik.com, the Admin Service frontend UI will be served at
https://traefik.com/${model-name}-${app-name}/ui

Right now the frontend application will be built with a base of ./. This allows the charm to work without knowing exactly where the Admin UI is served, but will break in case any other url is used with a trailing slash.
For example /ui/providers/ will make it so the assets are requested at /ui/providers/assets/....

To fix this, the Frontend will need a way to know which context-path/prefix to use when downloading assets, in order to not have a blank page when the react app is downloaded.

@huwshimi suggested we aim at a template processing solution, where the index.html will have one or more variables to handle the needed prefix for assets, and the backend will populate those variable by processing the template.