Introduce flag in cardano-testnet create-env to update time stamps

[nbacquey]

Description

This PR adds a helper to avoid manually updating time stamps in byron-genesis.json and shelley-genesis.json before starting a testnet with a modified configuration

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. These may include:
  • integration tests
• Any changes are noted in the CHANGELOG.md for affected package
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  • Code builds on Linux, MacOS and Windows for ghc-9.6 and ghc-9.12
• Self-reviewed the diff

Note on CI

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[Jimbo4350]

This looks good to me but I think this should exist as a separate command. We have experience with this in cardano-cli where too many flags are added to a single command making it unwieldly.