feat(fluent-api): create fluent builder API

- implemented builders:
  - StartEvent- and EndEventBuilder
  - UserTask-, ServiceTask- and ScriptTaskBuilder
  - ParallelGateway- and ExclusiveGatewayBuilder
- add example test for invoice process
- partially implement activiti custom attributes:
  - formKey
  - assignee
  - candidateUsers
  - candidateGroups
  - class

related to #CAM-1706

feat(fluent-api): create fluent api architectur

feat(fluent-api): add more builder and test cases

feat(fluent-api): add invoice process example as test

Add activiti custom attributes