Feat: use the default workspace config when the specified workspace is not found

[liu-hm19]

What would you like to be added?

The user-specified workspaces (e.g. dev, pre or prod) currently need to exist to perform the related operation like kusion generate and kusion apply, please add support for the default workspace so that kusion can use the default workspace config (default.yaml) when the user-specified workspace is not found.

Why is this needed?

This can bring more convenience for the kusion cli users.