plugins: support plugin specific paths, adding a plugin directly (#301024)

* plugins: support plugin specific paths, adding a plugin directly

- Supports custom fields for commands/skills/agents/etc in plugins,
  matching the new plugins spec.
- Support adding a plugin (without needing a repo) directly via
  'Chat: Install Plugin from Source' in the command palette, installed
  plugins view, or the chat customizations view.

Closes https://github.com/microsoft/vscode/issues/300945?reload=1

* comments