feat: support configurable measurands per transaction stage in OCPP 2.0

feat: support configurable measurands per transaction stage in OCPP 2.0

- Thread measurandsKey and context params through the meter value pipeline
  (getSampledValueTemplate, build*MeasurandValue, voltage helpers)
- buildTransactionStartedMeterValues uses SampledDataCtrlr.TxStartedMeasurands
  with Transaction.Begin context
- buildTransactionEndedMeterValues uses SampledDataCtrlr.TxEndedMeasurands
  with Transaction.End context
- Add OCPP 1.6→2.0 mappings for MeterValuesAlignedData,
  ClockAlignedDataInterval, StopTxnSampledData, StopTxnAlignedData
- Reorder getSampledValueTemplate params: measurandsKey before measurand
- Log warn (not debug) when meter value building fails in transaction events
- Add .trim() to convertToBoolean for whitespace-padded values