Timer block configuration via VC document fields

### Problem description
Currently values for CRON, Start Date, End Date in the timer block are set statically when the policy is created and cannot be changed. This is too constraining, there should be more flexibility in dynamically configuring these during the Policy execution time.

### Requirements

- Introduce the capability to configure CRON, Start Date, End Date in the timer block based on the values from the field in a VC document.
- Add the facility to map the fields of VC documents to settings in the timerBlock
- Make sure Policy Modules support this capability.

### Definition of done

- Functionality is implemented as per above
- An example is introduced into one of the sample Policies
- Documentation updated accordingly

### Acceptance criteria
TimerBlock can be configured at Policy run-time based on the values from VC documents.