feat(rest): fetch serialized values custom object variables

- GET /variable-instance query returns serialized value for all variable
  types that store custom objects, except the Serializable type that returns
  no value at all
- GET /variable-instance/{id}/data returns the binary representation of
  serializable variables
- same for historic-variable-instance and historic-detail
- PersistentVariableInstance has two new methods:
    - storesCustomObjects(): indicates whether this variable is potentially
    a non-JDK value
    - getValueTypeName(): returns the simple class name of the variable value
    on a best effort basis

related to CAM-2435