create-testnet-data: move Byron handling to its own file

[smelc]

Changelog

- description: |
    create-testnet-data: move Byron handling to its own file
# uncomment types applicable to the change:
  type:
  # - feature        # introduces a new feature
  # - breaking       # the API has changed in a breaking way
  # - compatible     # the API has changed but is non-breaking
  # - optimisation   # measurable performance improvements
  - refactoring    # QoL changes
  # - bugfix         # fixes a defect
  # - test           # fixes/modifies tests
  # - maintenance    # not directly related to the code
  # - release        # related to a new release preparation
  # - documentation  # change in code docs, haddocks...

Context

• Follow-up of create-testnet-data: create byron genesis #974
  • I know more documentation was asked in 974, but I'd need to dig the node's implementation to obtain it; and I'm not sure it's a worthwhile usage of my time, given that Byron will eventually be phased out. Reviewers> LMK if you think differently.
• Fixes create-testnet-data: Follow-up of byron genesis creation #976

How to trust this PR

This is just code moved around: no functional change.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• Self-reviewed the diff

[palas]

Looks good 👍