[v6] Custom Registry Support for tfx-cli

[jessehouwing]

Scope: Allow specifying alternative npm registry + auth for tfx-cli install.
Acceptance Criteria:

• Inputs tfxRegistry / tfxRegistryAuth functional.
• Installation falls back to default if unreachable custom registry.
• Auth handled for Azure Artifacts feed.
  Tasks:
• Implement registry config writer (.npmrc).
• Add auth token injection.
• Fallback logic & tests.
  Labels: enhancement