Consider adding overzoom property to document maximum overzoom level

[samanpwbb]

Some tile server implementations support overzooming, where tiles are generated and served at zoom levels beyond a tileset's zoom extent using the lowest available zoom level as the source of truth for overzoomed tiles.

Mapbox's tile server includes an overzoom max of 30 for all vector tilesets, which means tiles can be requested from any tileset from the minzoom to zoom 30, regardless of the maxzoom. Requesting tiles beyond z30 from Mapbox results in a 422 error.

If overzoom were a property on tileJSON, clients would know when to stop requesting tiles without hitting an error or relying on a separate source of truth.