Introduce new public auth mode for single shop tenant

[alanko0511]

This PR introduces a new authentication mode, publicShopTenant, that allows apps like Shopify theme app extensions to set up an unauthenticated role with a Shopify tenancy.

This allows use cases like fetching publicly accessible data for the specific shop without manually passing in the shop ID.

Click here to see an example usage

Example usage:

<!-- In extensions/theme-extension/blocks/star_rating.liquid -->
<script src="https://shop-auth-mode--development.ggt.pub/api/client/web.min.js" defer="defer"></script>
<script>
document.addEventListener("DOMContentLoaded", function () {
  window.gadgetPublicClient = new Gadget({
    authenticationMode: {
      publicShopTenant: {
        shopId: {{ shop.id }},
      },
    },
  });
})
</script>
<script src="{{ 'getShopInfo.js' | asset_url }}" defer="defer"></script>

// In extensions/theme-extension/assets/getShopInfo.js
const fetchCurrentShop = async () => {
  const shopInfoComponent = document.getElementById("shop-info");
  const shopInfo = await window.gadgetPublicClient.getCurrentShop();
  if (shopInfoComponent) {
    shopInfoComponent.innerHTML = JSON.stringify(shopInfo, null, 2);
  }
};

document.addEventListener("DOMContentLoaded", function() {
  fetchCurrentShop();
});

Then the shopInfoComponent should render the payload from the getCurrentShop global action.

PR Checklist

• Important or complicated code is tested
• Any user facing changes are documented (changelog section below)
• Any app Problems are added to the problem finders
• Any app data that needs a redeploy to take effect is listed in the contentHash functions
• Important or complicated production behaviour has traces and logs added

Changelog

If this PR will affect change user-facing behaviour, add detailed information about the change here to prompt an AI generated changelog entry.

If no changelog is required, write no-changelog-required in square brackets [].

[infiton]

let's make sure we agree on best name for this