feat(engine): add #getSerializedValue for variables

- extends interfaces VariableInstance, HistoricVariableInstance,
  HistoricVariableUpdate
- introduces interface SerializedVariableValue that allows to access
  the serialized value of a variable as well as additional type-specific
  configuration parameters

related to CAM-2544
refactor(engine): expose getVariableInstance methods in VariableScope

related to CAM-2544
feat(engine): set variables in their serialized form

- adds interface PersistentVariableScope that allows to set variables
  by providing their serialized value
- extends RuntimeService by methods to set variables in their serialized
  form

related to CAM-2544