Support authenticated Agent Plugin installation from private repositories #55775

• Follow up to Add pinned Agent Plugins support #54288

Private Agent Plugin marketplaces are common in enterprises. Could plugins: offer opt-in per-plugin credentials, similar to skills:?

plugins:
  - plugin: octo-org/private-plugin@main
    github-token: ${{ secrets.PRIVATE_PLUGIN_TOKEN }}

  - plugin: octo-org/private-marketplace/plugins/example@main
    github-app:
      client-id: ${{ vars.PLUGIN_APP_CLIENT_ID }}
      private-key: ${{ secrets.PLUGIN_APP_PRIVATE_KEY }}

Expected behavior:

• Public plugins continue to work without configuration.
• Existing string/array syntax works as expected
• github-token and github-app are optional and mutually exclusive per plugin.
• At runtime, the configured credential is used only for that plugin's pinned checkout.

I'm not sure how you could handle this for local compilation. Right now, local compilation attempts to access the plugin. I imagine we don't want the compiler to attempt to evaluate workflow token expressions or GitHub App credentials.

Ideally you would retain the behavior introduced in #54288 while enabling private plugin distribution.