add: get_custody_columns for das_core

[agnxsh]

This Pr adds get_custody_columns and it's adjoining helper functions, along with networking fixture tests provided by EF

Ref:

• https://github.com/ethereum/consensus-specs/blob/v1.5.0-alpha.5/specs/_features/eip7594/das-core.md#get_custody_columns
• https://github.com/ethereum/consensus-specs/blob/v1.5.0-alpha.5/tests/formats/networking/get_custody_columns.md

[github-actions]

Unit Test Results

         9 files  ±  0    1 343 suites  +3   42m 3s ⏱️ - 2m 7s
  5 140 tests +18    4 792 ✔️ +18  348 💤 ±0  0 ❌ ±0 
21 192 runs  +27  20 788 ✔️ +27  404 💤 ±0  0 ❌ ±0 

Results for commit f66ffdd. ± Comparison against base commit 8306ad5.

♻️ This comment has been updated with latest results.