ui-chart: allow changing series name when using JSON type

### Description

The [documentation](https://dashboard.flowfuse.com/nodes/widgets/ui-chart.html#series) says:

> Alternatively, you can set the series property to type JSON, and then provide an array of keys (e.g. ["key1", "key2"]), which will plot a data point for each key provided, from a single data point.

It would be nice to be able to change the names of the series, as the keys are usually "ugly".
I think it could be solved allowing the property to be a JSON object, with the keys being the object keys, and each value the desired series name, such as:
```json
{
"value": "Series 1",
"nested.value": "Series 2"
}
```

### Have you provided an initial effort estimate for this issue?

I can not provide an initial effort estimate