Use a struct for namespace

[rach-id]

          Hmmm. Should this instead be a struct of a `bytes1` and `bytes28` https://github.com/celestiaorg/celestia-app/blob/3e6d22cfde678efac4f7c078228e9f294a7a9b8d/specs/src/specs/namespace.md#overview? The actual namespace ID isn't just one field. There isn't any code in the QGB proper that cares, but perhaps downstream contracts might care (e.g. to ensure their data was posted to the correct namespace).

Doing that would increase costs though.

Originally posted by @adlerjohn in #171 (comment)