Create STAC extension for RDLS

[stufraser1]

Create an extension to STAC to incorporate RDLS metadata.
Need to consider: one extension for RDLS, all data types within; or one extension per RDLS data type (data type = hazard, exposure, vulnerability, loss)

STAC extensions are made available here: https://github.com/stac-extensions

Initial check: create an item with the existing RDLS metadata objects and strings, and asset, which is what we call resource

Proposal - create a pilot extension with exposure, as the most simple of the data types.
Later extend with hazard, vuln, loss.

Also propose creating a catalogue / collection to link them all - equivalent to WB Data Catalog RDL Collection