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[bot]]

Unit Test Results

         9 files  ±  0    1 343 suites  +3   42m 3s :stopwatch: - 2m 7s   5 140 tests +18    4 792 :heavy_check_mark: +18  348 :zzz: ±0  0 :x: ±0  21 192 runs  +27  20 788 :heavy_check_mark: +27  404 :zzz: ±0  0 :x: ±0 

Results for commit f66ffdd6. ± Comparison against base commit 8306ad58.

:recycle: This comment has been updated with latest results.