Resolve input variable schema for multi-instance activities

[marstamm]

Is your feature request related to a problem? Please describe.

Follow-up from #2

Describe the solution you'd like

• When I create a multi-instance activity, I want the InputElement populated with the schema for entries derived from the input collection
• This should be available in the Multi-instance element and all sub-elements in case of a multi-instance subprocess

Describe alternatives you've considered

Additional context

related to https://github.com/bpmn-io/internal-docs/issues/680