da: refactor to use abstract da interface

[tuxcanfly]

Description

This PR adds a new pkg da, which abstracts the Data Availability layer so that it be customized without changing the implementation a lot.

Tests

Existing tests for e.g. calldata_source_test.go have been updated to work with new pkg.

Invariants

No new assumptions are made, the changes are purely cosmetic in terms of the architecture.

Additional context

There is still work to be done to integrate the new pkg with the older changes related to changing the data layer to celestia.