Add links to other geospatial representations

[jabhay]

PSMA is currently pushing all of its data out through APIs. Capability to link to other geospatial representations, e.g., lines for street, polygons for locality would be great.

The first stage in this is for the mechanism to link to them to be established, the next stage would be to enable the links themselves. Localities are currently published through NationalMap's WFS service, so can already be linked to. Street would require PSMA to publish a corresponding endpoint.