Introduce concept of "context"

[gtema]

Currently users need to mess around with the clouds.yaml and anchors to reuse authentication when needing a dedicated "context" with the same credentials, but different scope (i.e. project_a, project_b, domain_a, etc).
A very similar issue is being addressed by kubernetes contexts which separates clusters and users. Docker also has the concept of contexts.
So an idea is to introduce contexts which can be switched like osc set context or also osc --os-context .... For backwards compatibility we need to keep clouds.yaml as it is, but we can reuse it adding new root keys similar to:

clouds:
  cloud_a:
    auth:
      auth_url: https://a.bar
      username: foo
      password: bar
      user_domain_name: some_domain
contexts:
  a_project_a:
    cloud: cloud_a   # pointer to the cloud connection (similar to the `cluster` in kubecontext)
    scope:
        project: project_a
  a_project_b:
    cloud: cloud_a   # pointer to the cloud connection (similar to the `cluster` in kubecontext)
    scope:
        project: project_b

Alternatively contexts could be integrated into ~/.config/osc/config.yaml but then it is not logical to access those from tui.

What are the general thoughts? I admit not many would have this problem as such, but I, as a developer with access to multiple clouds, have it daily. With ostui we already move closer to the kubernetes world, so this looks like a logical next step.