Implement additional Extractors

[pinussilvestrus]

On top of the existing camunda:OutputParameter extractor, this adds additional extractors for

• form fields (user tasks, start events)
• result variables (service task like elements; like business rule tasks, ....; script tasks)
• out mappings (call activities)

Which issue does this PR address?

Closes #3

Acceptance Criteria

• [ ] Corresponds to the concept
• [ ] Corresponds to the design

Definition of Done

• [ ] corresponds to the design principles
• [ ] corresponds to the code standards
• [ ] passes Continuous Integration checks
• [ ] available as feature branch on GitHub
  • [ ] contains the cleaned up commit history
  • [ ] commit messages satisfy our commit message guidelines
  • [ ] a single commit closes the issue via Closes #issuenr

[pinussilvestrus]

tbd: do we also want to handle Call Activity camunda:out properties? https://docs.camunda.org/manual/latest/reference/bpmn20/subprocesses/call-activity/#passing-variables