[Feature] Custom action mapping (IKEA E1743)
**Problem description**
Currently my IKEA E1743 exposes four actions to this plugin: on, off, brightness_move_up and brightness_move_down. These are interpreted as four separate buttons, even though the brightness up and down buttons are the actions for long pressing the buttons.

**Suggested solution**
We could either modify the SwitchActionHelper in `src/converters/action_helper.ts` to recognise these brightness commands, but that would likely result in some ugly code and possible interference with other devices.

A better solution in my opinion would be to add a new config property to the `devices` array that allows you to map certain properties in the MQTT messages to different values. The syntax would be similar to the `values` property.

Example:
```json
{
    "devices": [
        {
            "id": "...",
            "mappings": [
                {
                    "property": "action",
                    "values": {
                        "on": "single_on",
                        "off": "single_off",
                        "brightness_move_up": "hold_on",
                        "brightness_move_down": "hold_off"
                    }
                }
            ]
        }
    ]
}
```

After these mappings are applied, the SwitchActionHelper would map these to the correct buttons.

**Describe alternatives you've considered**
Having all the actions as separate buttons is doable, but this would be my preference.

**Additional context**
n/a

**Are you willing to contribute to this feature?**
Yes, I'd be able to work on a PR if a maintainer approves of this feature.
