Request: config option to change static ui path prefix

### What would you like to be added?

Add an option such as `ui.prefix` to the firefly-core config options here:
https://hyperledger.github.io/firefly/latest/reference/config/#ui

### Why is this needed?

The Firefly UI (easily accessibly via http://firefly:5000/ui) is not accessible once its put behind a reverse proxy with a path prefix.

For example if it was necessary to access the ui behind https://example.com/firefly/ui, the browser would make requests to static resources without the `/firefly` path prefix resulting in 404s at https://example.com/ui/static/js/main.js.

A configuration option that allowed you to either provide the entire FQDN+prefix or just the path prefix where the service sits would solve this problem. I was hoping that `api.publicURL` config option would serve this purpose but it does not.

It may not be possible for all users to place the firefly ui/service directly at the root prefix.