Make `get-hax-ref.yml` into a re-usable action

In mainline libcrux, we have the workflow [`get-hax-ref.yml`](https://github.com/cryspen/libcrux/blob/5a1d172a1bcff83bb401bfa718d08a2dc8c77e4e/.github/workflows/get-hax-ref.yml) for retrieving the version of hax that should be used for extraction from the `cargo metadata` output about `hax-lib`. Now that we have a similar setup growing in `libcrux-iot` it makes sense to make that into a proper reusable action.