Update add-chain script to handle input variations within genesis.json and rollup.json

[bitwiseguy]

Currently the add-chain go module expects a single, rigid format of the genesis.json and rollup.json files that it reads from the monorepo. We should add code and tests to handle acceptable variations within those files.

** Additional context **

• Notion doc describing genesis changes that impact the registry-data tool
• Github thread discussing the issue

[geoknee]

This work is partially complete, but we need to extend it to cover custom gas token config.