Support listing of base feature sources

Allow the client to list all of the base feature sources or just the sources near a point.

Eventually we will use a separate table to manage the metadata for our base geodata. Then we can better define what "near a point" means and provide a real description string. For now, "near a point" means all sources with some features that intersects a bounding box centered at the specified point with height and width of 0.14&deg;.

/cc @hampelm