`edgedb configure` should maybe get the list of config values via reflection

Otherwise we'll probably end up always missing some.
If we do it via reflection, it's easy to include extension configs too.

This should get all the config properties that can be set with SET
```
select schema::ObjectType {
name,
properties: { name, typ := .target.name },
}
filter .name = 'cfg::AbstractConfig'
or 'cfg::ExtensionConfig' in .ancestors.name;
```