Allow non unit strides in DASFFTExtension()

Hardcoding the value `1` for the initial stride [here](https://github.com/protolambda/go-kzg/blob/98afb8f0b71f307672e1c06fd4d69414dfbb078b/das_extension.go#L75) is unnecessarily restrictive:

```
	fs.dASFFTExtension(vals, 1)
```

See https://github.com/benjaminion/c-kzg/issues/10