Support a second `config.yml` file for 'local' configuration

### Description

Currently the device agent picks up its configuration from command-line params and `device.yml`.

There are some options the user may want to set in their `device.yml` that do not come from the platform. This means the user has to merge changes in their device.yml file which could be error prone.

This includes:
- `port`
- `interval`
- `verbose`
- `moduleCache`

With #92 we are adding a bunch more CLI params for the config UI.

The proposal here is to support a second config file - `config.yml` that can be used to set any option not otherwise provided by the platform's `device.yml`.

When starting, the agent would load `config.yml` (if it exists), then apply `device.yml` (if it exists) over the top, and then apply any CLI param overrides.

### Epic/Story

_No response_