Exporters port config - common behaviour

## Problem

Currently, the charm config UX is uneven between the exporters. An operator can configure a por for `hardware` and `smarctl` exporters, but not for the `dcgm` one.

## Solution

We could either:

- Add support for port configuration for the `dcgm` exporter.
- Drop the port configuration for all exporters.

Personally, I prefer the second option as it will drastically simplify the charm logic. Also, it seems like this option is rarely used in production.

Start by comparing how port configuration is handled for the hardware, smartctl, and dcgm exporters in the charm. Review the existing comments and configuration paths, then confirm which common behavior is intended; done means all exporters expose consistent port behavior and the charm logic and tests reflect that choice.

Written by the indexing model from the issue text.