extend thebe option for pipilite urls

[stevejpurves]

We currently hard code piplite urls here: https://github.com/executablebooks/thebe/blob/cce89f9051b750145081e78ed7cf8deff24ae14e/packages/react/src/ThebeServerProvider.tsx#L95

These should be defaults that can be overridden via the options objects

[stevejpurves]

here's an example of a lite option set currently in use in the simple demos

      {
        "litePluginSettings": {
          "@jupyterlite/pyodide-kernel-extension:kernel": {
            "pipliteUrls": ["https://unpkg.com/@jupyterlite/pyodide-kernel@0.2.0/pypi/all.json"],
            "pipliteWheelUrl": "https://unpkg.com/@jupyterlite/pyodide-kernel@0.2.0/pypi/piplite-0.2.0-py3-none-any.whl"
          }
        },
        "enableMemoryStorage": true,
        "settingsStorageDrivers": ["memoryStorageDriver"]
      }

There is probably more to add here in order to introduce the service worker overrides now available to us in jupyterlite