front: create a simple StdcmPathStep type

[clarani]

Add a new stdcmPathSteps props in the stdcm reducer and create a new StdcmPathStep type (which extends PathStep for the moment):

export type StdcmPathStep = PathStep & {
  isVia: boolean;
  tolerances?: { before: number; after: number };
} & ({ isVia: true; stopType: StdcmStopTypes } | { isVia: false; arrivalType: ArrivalTimeTypes });

This will simplify the stdcm interface.

Will be followed by #9462