Question: does command gsutil support local testing with STORAGE_EMULATOR_HOST env

In https://cloud.google.com/go/docs/reference/cloud.google.com/go/storage/latest. they states that you can use `STORAGE_EMULATOR_HOST` to point to your local env.

Does `gsutil` suport this env? or do we have any way to customize the endpoint URL

```sh
export STORAGE_EMULATOR_HOST=localhost:4443
gsutil -d ls
```

Note that aws cli can specify endpoint by `aws s3 ls --endpoint-url=xxxxx`

Start with the `gsutil -d ls` invocation and the linked Cloud Storage client reference. Determine whether `STORAGE_EMULATOR_HOST` is honored and whether gsutil exposes another endpoint customization mechanism; done means a documented answer or a clearly scoped implementation path.

Written by the indexing model from the issue text.