Implement spatial extent filter support in API V2

[Samweli]

Looking at the API V2 implementation here GeoNode/geonode@44470b9 it seems the spatial extent filter can be used by passing extent parameter in the API V2 layers fetching endpoint without using the filter{} parameter.

example
https://master.demo.geonode.org/api/v2/layers?extent=-10,-10,-1,-1

returned layers

{
    "links": {
        "next": "https://master.demo.geonode.org/api/v2/layers?extent=-10%2C-10%2C-1%2C-1&page=2",
        "previous": null
    },
    "total": 79,
    "page": 1,
    "page_size": 10,
    "layers": [
        {...