Support passing a custom duration to /schedules/trigger [POST]

[Flix6x]

Add an optional duration parameter to the schema, instead of always falling back to FLEXMEASURES_PLANNING_HORIZON.

[Flix6x]

Connected to this, we'd like to allow SoC targets to exceed the default scheduling horizon, dynamically extending the planning window.

Perhaps we should make a distinction in the config settings: between FLEXMEASURES_DEFAULT_PLANNING_HORIZON (a fallback in case no duration is specified nor can it be derived from the future-most target) and FLEXMEASURES_MAX_PLANNING_HORIZON (allowing FlexMeasures hosts to limit the computational effort, although that also depends on the sensor resolution).