line-gradient for vector tile sources

Nobody has claimed this yet.

## Motivation

I would like to use the `gradient-line` on a vector tile source.
For now, this option can only be used with GeoJSON sources. But this type of source is not really usable for large datasets.

## Design Alternatives

## Design

### Mock-Up

### Concepts

### Implementation

The API wouldn't change :

```
map.addLayer({
"id": "line",
"type": "line",
"source": {
type: 'vector',
url: 'mapbox://mapbox.mapbox-terrain-v2'
},
"source-layer": "line",
"layout": {
"line-cap": "round",
"line-join": "round"
},
"paint": {
"line-opacity": 0.6,
"line-width": 0.01,
'line-gradient': [
'interpolate',
['linear'],
['line-progress'],
0, "blue",
1, "red"
]
}
});
```

Start with the map.addLayer example in the issue and trace the existing gradient-line support for GeoJSON sources. Determine what must change for the same API to work with vector tile sources, and consider the relevant implementation and edge cases before defining completion.

Written by the indexing model from the issue text.