TransformRequest function to allow Custom Auth for each request

[james-willis]

In the zarr-gl and zarr-layer projects, we want to allow users to request data that requires authentication, for example from s3 using presigned URLs. We accomplish this by letting them provide a transformRequest method like that of MapLibre: https://maplibre.org/maplibre-gl-js/docs/API/type-aliases/RequestParameters/

We have a PR to do that in zarr-layer but we think having zarrita support some kind of fetchFn parameter might be a good idea for other zarrita users. It would also simplify the code in zarr-layer for this feature. PR for zarr-layer support: carbonplan/zarr-layer#6

zarr-js sovles this by allowing the passing of a custom fetch function when creating a store:

zarr(fetchFn, "v3").open(
  `${source}/${level}/${variable}`,
  (err: Error, get: Loader) => {
    if (err) {
      reject(err);
      return;
    }
    resolve([`${level}/${variable}`, get]);
  },
);

where fetchFn implements the interface of window.fetch

[keller-mark]

The fetch store supports passing options for fetch requests https://github.com/manzt/zarrita.js/blob/main/packages/@zarrita-storage/src/fetch.ts#L68

[james-willis]

passing options is not sufficient for e.g. generating signed s3 URLs, you need a function that takes in the request address (e.g. s3://bucketName/prefix.zarr/zarr.json) and outputs a signed url. The signed URL is different for each object so we cant generate it ahead of time and provide it as a string input