cardano-testnet: harmonize node and genesis files API + allow (for example) to only pass a custom conway genesis

[smelc]

Description

• Work towards implementing option 3. from cardano-testnet: choose a design to support user-provided files #6137: we support a mix of generated genesis files and genesis files provided by the user.
• Follow-up of problem reported by user: [BUG] - cardano-testnet doesn't reflect committee members added into Conway genesis configuration #6130 (comment)

Together with #6103 (6103 being stacked on this PR), this PR allows for the following:

• Having cardano-testnet usable as a standalone executable
• Allowing overriding 0, 1, or many genesis files. If a user doesn't specify a genesis file, cardano-testnet generates it. This is exemplified in cardano-testnet executable: take node config file as optional input #6103 test of the cardano-testnet executable, that specifies only the Alonzo and Conway genesis files, but leaves the Shelley genesis to be created automatically: https://github.com/IntersectMBO/cardano-node/blob/smelc/testnet-pass-config-files/cardano-testnet/test/cardano-testnet-test/Cardano/Testnet/Test/CommandLineExecutable.hs#L33
• This is done by passing (or not) custom genesis files. If a file is omitted, it's generated. There's no need for corresponding flags.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated ↔️ Tested in cardano-testnet executable: take node config file as optional input #6103
• CI passes. See note on CI. The following CI checks are required:
• Self-reviewed the diff

[smelc]

@IntersectMBO/performance-and-tracing> this PR removes the creation of the genesis spec files. Given that we now have a complex mechanism for genesis files being passed by the user or generated on demand, maintaining the creation of spec files is too complex on our side.