feat(engine-rest): Add local variables methods to task rest service

GET /task/{id}/localVariables
    returns all local variables for a task

GET /task/{id}/localVariables/{varId}
    returns the local variable for the given task and variable name

PUT /task/{id}/localVariables/{varId}
    set a new local variable to the given task with the variable name
    from the path parameter 'varId'

POST /task/{id}/localVariables/{varId}/data
    set a new local binary variable to the given task id with the variable
    name from the path parameter 'varId'

DELETE /task/{id}/localVariables/{varId}
    delete the given variable from the given task

POST /task/{id}/localVariables
    update/delete the given variables

- extends TaskServiceImpl with the 'updateVariablesLocal' method
- add testcase to TaskServiceTest for method 'updateVariablesLocal'
- add some testcases for rest api extension

related to #CAM-2141