config: provide context support

[gfelbing]

As a stackit user, I usually deal with multiple projects (e.g. for staging) and configurations. Currently, only a single configuration is available via ~/.stackit/cli-config.json.

Proposal: Add a --context flag to support multiple configurations. Those can be based on configurations like

// ~/.stackit/cli-config.json
{
  "default_context": "my-default-context"
  // ...other settings (global only / defaults for all contexts)
}

// ~/.stackit/contexts.d/my-default-context.json
{
  "project_id": "<project_id>",
  // ...other settings specific for this context
}