Progress tracker for index creation

Index creation can be very expensive for large CARs and it'd be nice to be able to track (and in the CLI emit) progress as it's being generated.

If https://github.com/ipld/go-car/blob/e48b3f16cfc2014e2e56dae952d04021ac66a61b/v2/index_gen.go#L43 took a context I'd probably recommend attaching the tracker there (and probably also allowing the caller to bail via context cancellation). We could potentially put it into the Options struct, but I don't know if that fits the model of what that struct is aiming for.

Any suggestions on acceptable API modifications (or new function introductions) here?