Multi-environment config via named contexts

[kichristensen]

What does this change

Adds kubectl-style multi-context support to the Porter config file. Users can define multiple named environments in a single config file and switch between them at runtime.

New schemaVersion: "2.0.0" format:

schemaVersion: "2.0.0"
current-context: default
contexts:
  - name: default
    config:
      namespace: dev
      default-storage: localdb
  - name: prod
    config:
      namespace: prod
      default-storage: proddb

Context selection (highest to lowest priority):

1. --context prod flag
2. PORTER_CONTEXT=prod env var
3. current-context: field in the config file
4. Falls back to a context named "default"

New commands:

$ porter config context list
* default
  prod

$ porter config context use prod
Switched to context "prod".

Legacy flat config files continue to work unchanged. Using --context with a legacy file is a hard error with a clear message.

What issue does it fix

Closes #3523

Notes for the reviewer

• Context list/use use viper internally so they work with any supported config format (YAML, TOML, JSON, HCL)
• ConfigContextUse does a targeted regex replace of the current-context: line to preserve Liquid template variables elsewhere in the file
• The defaultConfigTemplate (used by porter config edit when no file exists) now generates the new versioned format

Checklist

• Did you write tests?
• Did you write documentation?
• Did you change porter.yaml or a storage document record? Update the corresponding schema file.
• If this is your first pull request, please add your name to the bottom of our Contributors list. Thank you for making Porter better! 🙇‍♀️