config command should allow for a tweaking config file values

[displague]

What problem are you facing?

After the metal config file has been created, it would be helpful if the file could be modified from the command line.

How could the Equinix Metal CLI help solve this problem?

metal config --token=... --facility=... --project-id=... --plan=...

While it is possible to accept any name and convert it to a config file entry, an exclusive list will prevent users from introducing typos or setting dangerous defaults (--id, --name, etc).