feat: implementing short namespace absence proof generation

# Problem

We currently have the necessary logic to verify short absence proofs (see https://github.com/celestiaorg/nmt/issues/210), yet the proof generation counterpart does not exist in the nmt library. This issue is to track the progress on the implementation of the proof generation component.

Start by reading the existing short absence proof verification logic in the nmt library and the discussion in issue #210. Identify the proof structures and generation entry points that verification expects. Done means the library can generate short namespace absence proofs that pass the existing verification logic.

Written by the indexing model from the issue text.