`port` configuration option is not configuring Dex's command

### Bug Description

Despite having a configuration option `port` for changing Dex's ["listening port"](https://github.com/canonical/dex-auth-operator/blob/main/config.yaml#L9), this value is not used at all by Dex's command; if users change this value via the config option, Dex will not use it at all, as it is not reflected in the service [`command`](https://github.com/canonical/dex-auth-operator/blob/main/src/charm.py#L88)

Also, calling it "listening port" is misleading, according to the documentation, it is "The HTTP port used by Dex, default is 5556. Used to define web -> http in the Dex configuration as well as OIDC configurations". The configuration description should be more clear about it.

We should change this by making the proper refactor in the code and in the rock.

Start with config.yaml and the service command in src/charm.py around line 88, then inspect the rock configuration referenced by the issue. Trace how the port option reaches Dex and update the command, rock, and configuration description so the configured HTTP port is used consistently.

Written by the indexing model from the issue text.