feat(scripts): support empty and mixed UTXO backends
- Add "empty" as a valid option for UTXO_BACKEND and MIXED_UTXO_BACKENDS in both common-start-fast and common-start-slow scripts. - Implement logic to rotate through mixed UTXO backends for block producing nodes, defaulting to "empty" (unset) when specified. - Update testnet.json documentation to clarify that the default backend is "empty" (mem without configuration) if unset.