Type safe layer sources

[sargunv]

Layers currently accept any Source (in the type system), but usually layers can accept one of: raster, raster-dem, or vector (tiles, geojson, computed, etc) sources. This should be enforced by the type system.

It's not trivial because we have UnknownSource.

[sargunv]

@cursoragent we added UnknownSource long before the recent architecture pivots. All of the underlying code has changed. Explore if the UnknownSource is still necessary, or if we can switch the api over to typed descriptors trivially today

[cursor]

Taking a look!