Multi-Shop support

Describe the feature

Currently it's not possible to configure more than one shop using the module. This is because we rely on the schema from only one store to get the API types. We could make multiple stores configurable and have each generate their own types, but that would be a big change and a much more complicated module config.

What we could do more easily though is add an optional shops config option, where different stores can be configured and then the shop that would be used would become a request header or setting in the api client:

export default definNuxtConfig({
  shopify: {
    name: 'my-store-name-for-codegen',
    clients: { storefront: { ...  } },

    shops: {
      shop1: { name: 'my-other-shop-1', clients: { ... } },
      shop2: { name: 'my-other-shop-2', clients: { ... },
    }
  }
})

Maybe not the most optimal solution but it would enable a true multi-store-setup at least.
For now it hasn't come up but i want to hold the idea here.

Start by tracing how the Shopify configuration, schema-based code generation, and API client are currently connected. Evaluate the proposed optional shops configuration and how the selected shop could reach the request header or API client setting. Done would mean multiple stores can be configured and selected without requiring separate generated types for each store.