Add option to lazy-extend ExtendedDataSquare

Instead of extending ExtendedDataSquare when imported, add an option to only extend certain rows or columns when the extended portion is accessed. This for use cases that do not need to read the extended square.

Start by locating ExtendedDataSquare and the import path that currently extends it, then trace how extended rows and columns are accessed. The change is done when callers can opt out of eager extension and only the requested rows or columns are extended on access, with behavior covered by relevant tests.

Written by the indexing model from the issue text.