Save TDs or their URLs where they were fetched from

[VigneshVSV]

User can also save TDs or the location where they fetched it from.

Rough Steps:

• Create a table with user keycloak-ID as primary key, URL of string type, TD of JSON type, a descriptive name as unique key, description as TEXT.
• When the user asks to save a thing, request a name (mandatory) and description (optional)
• As usual, PATCH existing THINGS, POST to create new things, and GET to fetch all things to display them (only name, description and URL), and GET when a particular thing is asked to be loaded along with the TD
• If a link was provided, try to load the TD from that location if possible. If not, use the existing TD which was in the DB. The TD is stored in the DB only as a backup or if no URL is provided. But if a new TD was available from the URL, silently without intruding, ask the user if they wish to overwrite the value in the DB. Overwrite it if yes.
• Add tests

blocked by #44 and #45