Fix tooling to generate a test ledger

[georgeee]

Update some scripts and runtime_genesis_ledger to be able to generate a runtime configuration for a custom testnet cluster.

Explain how you tested your changes:

• Followed the steps to generate a runtime config with a custom genesis

Checklist:

• [x] Dependency versions are unchanged
  • Notify Velocity team if dependencies must change in CI
• [ ] Modified the current draft of release notes with details on what is completed or incomplete within this project
• [x] Document code purpose, how to use it
  • Mention expected invariants, implicit constraints
• [x] Tests were added for the new behavior
  • Document test purpose, significance of failures
  • Test names should reflect their purpose
• [x] All tests pass (CI will check this if you didn't)
• [x] Serialized types are in stable-versioned modules
• [x] Does this close issues? None