Implement Basic Spatial Functions conformance class from CQL2 for Filter Extension support

Currently, the basic spatial functions (e.g., s_intersects over the Item geometry) is not supported. Support for this should be added. See https://docs.ogc.org/is/21-065r2/21-065r2.html#rc_basic-spatial-functions-plus for conformance class details.

Note that the Filter Extension spec has not yet been updated to define the released version of CQL2, but the functionality is the same.