CRS Extension

[philvarner]

• Add CRS extension radiantearth/stac-api-spec#85

From @cholmes

We should add CRS as an extension, since it's the most mature OAFeat extension - https://portal.ogc.org/files/91976

Could just scope it to ogcapi-features and just add it in extensions.md. Or could also add it to item-search.

Related:

• CRS handling for the intersects parameter radiantearth/stac-api-spec#307