Support using a computed value for the remote changed time

In some synchronizations, the time that a remote entity was last changed is saved into a Drupal field or field property. In some cases that time is provided in an unsupported format e.g. wrapped in an array etc. We can support such cases by allowing to fetch the time from a computed value instead of directly getting it from a remote entity property. Computed values provide extensive abilities in transforming any property from the remote entity in practicality any way required.