Support more terrain encoding formats (TerrainRGB, Terrarium) + public terrain sources

[jo-chemla]

It's great being able to load a terrain of:

• a user-provided geotiff terrain DTM
• SRTM30, dynamically applied to exported satellite raster plane
• or user-provided custom WMS sources like shown in this comment Custom WMS as elevation  #893 (comment) with for example ("https://data.geopf.fr/wms-r/wms?SERVICE=WMS&VERSION=1.3.0&REQUEST=GetMap&STYLES=normal&FORMAT=image/geotiff&LAYERS=ELEVATION.ELEVATIONGRIDCOVERAGE.HIGHRES&WIDTH=5000&HEIGHT=5000&CRS=EPSG:4326&BBOX={S},{W},{N},{E}", 'Geoplateforme', 'Elevation data from IGN')

The custom terrain sources can be bbox bounds templated string with {W} {S} {N} {E} templates. It does not seem that one can input standard WebMercator EPSG:3857 {z}, {x}, {y} templates - which would anyway not be meaningful without the tileResolution control, usually 256 or 512.

It would also be great to add support for other terrain encodings well used for serving terrain tiles. Especially, terrainRGB and terrarium.

• https://docs.mapbox.com/data/tilesets/reference/mapbox-terrain-rgb-v1/TerrainRGB height = -10000 + ((R * 256 * 256 + G * 256 + B) * 0.1)
• Terrarium expression=(b1 * 256 + b2 + b3 / 256) - 32768
• quantized-mesh terrain spec

I've added in the details section below some sources for elevation that use the TerrainRGB or Terrarium - some public and some behind API key. It could be useful to add support for TerrainRGB which most sources use, as well as Terrarium - quantized mesh is a different beast altogether, probably too complex to implement.

Terrain Tiles Sources

• Mapterhorn great collection of national terrain sources: , see planet.pmtiles, xyz endpoint, TileJSON endpoint
  Also see aditional possible terrain data and wiesehahn + lidar-availability requests
• MapTiler TerrainRGB : XYZ endpoint and tileJSON endpoint. https://api.maptiler.com/tiles/terrain-rgb-v2/{z}/{x}/{y}.webp?key={KEY}
• AWS S3 Elevation Tiles: XYZ endpoint https://s3.amazonaws.com/elevation-tiles-prod/terrarium/${z}/${x}/${y}.png
• Mapbox terrain rgb: Mapbox TerrainRGB endpoint https://api.mapbox.com/raster/v1/mapbox.mapbox-terrain-dem-v1/${z}/${x}/${y}.webp?sku=101UGFSzooBV0&access_token={KEY}
• Using these datasets Access elevation data and mapbox-terrain-dem-v1.
• Mapzen: See blog post and doc, TMS looks like this https://tile.mapzen.com/mapzen/terrain/v1/terrarium/{z}/{x}/{y}.png