feat: allow custom metadata files

```py
download_file(
    name = "example",
    metadata = {
        "//:integrity.tmpl.bzl", ".integrity.bzl",
    },
)
```

Where `integrity.tmpl.bzl` is:

```py
visibility("public")

INTEGRITY = "{{integrity}}"
```

Then `@example//:.integrity.bzl"` can be used in `load` statement to read the download integrity.