Store workspace id in `mergin.json`

[uclaros]

If renaming of workspaces is to be implemented, we will need to stop relying on the namespace stored in mergin.json and use the workspace id. We should probably start storing it from the beginning even if we don't use it yet.
Maybe also store the server? Or even list of pairs server - workspace id?

[wonder-sk]

Probably it would be good if the project info endpoint included:

• project ID
• server URL
• workspace ID

That way we would get all that included in mergin.json automatically (rather than adding that extra within the client), and it could be useful also elsewhere (e.g. when trying to link an existing project checkout)

[wonder-sk]

Project ID gets stored in >= 0.8.0 ... see #154

[wonder-sk]

We have project ID and workspace ID stored in mergin.json since #83. The server URL is not stored anywhere at this point, but this could be added later if needed (working in the same way as the c++ client) - possibly in a separate file.