project: read `cloud_provider` from stack spec

[tormath1]

To keep a trace:

For the BE, we need to provide the --usecase and the --cloud-provider to the environment if we want to associate a cloud-provider to an environment.

From a naive approach, it looks like this:

cy project create --name my-project --usecase second --cloud-provider google ...

but my second provider defines a cloud_provider: aws so my final env will be associated to google.

The goal of this issue, is to be able to fetch the cloud_provider from the stack spec.

GET ​/organizations​/{organization_canonical}​/service_catalogs​/{service_catalog_ref}​/config

can be a good entrypoint for the implementation