Refactor DAH generation to give the user more control over what happens to the EDS

[evan-forbes]

Summary

Currently the extended data square is being garbage collected after it is created during the generation of a new DataAvailabilityHeader. We should make it so that this decision is not forced on the user.

Details

The user (celestia-node) might want to save the extended data square, so we should change how we generate DataAvailabilityHeaders to allow that to occur

Action Items

• Break up NewDataAvailabilityHeader func into two pieces

[liamsi]

Am I understanding this correctly: this is simply about exposing a method that can be used as a library in Celestia node? Or is there more to it?

[evan-forbes]

whoops, must have missed this comment, but I think we covered it in #539

also can close w/ the merge of #534