Terrain exaggeration: supporting data-driven styling

Nobody has claimed this yet.

## Motivation

I would like to be able to customise exaggeration based on elevation ranges, similar to what is currently possible with fill-extrusion layers (esp. height/colour).

I work with reef bathymetry data (@30-100m x/y resolution) that generally sees depth profiles between 0 to 60m. The reefs often sit on narrow shelfs, bordered by mountains, that quickly drop off to kms depth on the ocean side. Being able to emphasise detail on the reef shelf, while tapering the extremes either side, is quite useful. Stepped/interpolated exaggeration based on the `z` value would allow this.

In the past I have encoded DEMs using a custom height profile. This was primarily to (selectively) squeeze as much detail as possible - in the right places - into an 8bit png, which was all that was supported for upload into Studio at the time. Being able to decode the profile effectively on the front-end (using data-driven styles) was vital, but had the added effect of affording range-specific exaggeration.

Here's an example using a fill extrusion layer with data-driven styles for height (and colour): https://lab.citizensgbr.org/census-map/

## Design Alternatives

The same can be achieved by custom-encoded DEMs alone, however the exaggeration approach is more flexible (no re-encoding large tilesets to make adjustments) and ensures cross-compatibility (supplied elevation values are correct / per standards).

## Design / Mock-Up

Developers / client side implementation would work the same as now, except elevation data would be queryable:
```js
// Pseudo code
map.setTerrain({
source: 'bog-standard-dem',
exaggeration: [
'interpolate',
['linear'],
['get', 'z'], // based on z value, same as accessing in fill-extrusion
0, 2.5,
100, 1.5,
],
});
```

No implementation files or tests are named. Start by reviewing the current terrain exaggeration API and the existing fill-extrusion data-driven styling behavior described in the issue; done should allow exaggeration to vary by elevation z using stepped or interpolated expressions while retaining standard elevation values.

Written by the indexing model from the issue text.