Make lister pre-compute its api url from its domain

Currently, the pattern follows something like:

• gitlab: https://$domain/api/v4
• gitea: https://$domain/api/v1/
• rest: https://$domain

It'd be interesting the lister only takes the domain and computes the url directly (adds https:// to the domain, and prefix the correct api endpoint on the lister type).

That'd simplify all scheduling commands (e.g. new forge to list, new add-forge-now requests to execute, ...).

Optionally, it could also do a pre-flight check to ensure it's able to communicate with such api/page first.