refactor: make DummyDA publicly available

[tzdybal]

Overview

DummyDA is a simple, in-memory, minimalistic DA implementation for test purposes. It is used to validate unit tests in this repo, but should be public, so it can be used as "mock" by projects using go-da (for example to replace mock da in rollkit).

Resolves #6

Checklist

• New and updated code has appropriate documentation
• New and updated code has new and/or updated testing
• Required CI checks are passing
• Visual proof for any user facing features like CLI or documentation updates
• Linked issues closed with keywords

[tuxcanfly]

Minor nits.