load_stac: explicitly request proj metadata from STAC API

[bossie]

Discovered in the context of #778.

load_stac interprets properties proj:epsg, proj:shape and proj:bbox in the STAC item metadata but by default STAC APIs, like our own, only tend to return the bare minimum of metadata to make it a valid STAC Item i.e. datetime.

We should therefore explicitly request these properties from the API to make use of them.

[bossie]

Works on staging with process graph from #778.