add a JSON definition for endpoint specifier keywords

this should partially fix #335. the remaining bits are to register the relative permeability keywords as the grid properties in `EclipseState` and to handle these in the `GridPropertyEndpointTableLookupInitializer` class. (the latter I don't know how it should be done, probably that's the domain of @osae.) Note that the ENKRVD table is already fully supported (modulo bugs).