[Directory Use Case] Add web thing to smart home hub

[benfrancis]

As a user I want to add a web thing to my smart home hub so that it can be monitored and controlled through my smart home software.

In this scenario a Thing Description for a web thing is already hosted elsewhere (e.g. self-hosted by a device or by a third party cloud service) and the user wants to add it to the directory hosted by their smart home hub so it can appear in their smart home dashboard and be monitored, controlled and automated along with all their other smart home devices. The user wants to add the existing web thing by its URL, which can then be fetched to retrieve its existing Thing Description and re-fetched in future in case the device metadata changes.

When clients fetch the Thing Description from the directory, the directory may either just point to the source URL (acting as a directory), or proxy the Thing Description to a new URL at a new origin (acting as a gateway).