Camunda-modeler: Connect message flows to call activities

[nikku]

__Is your feature request related to a problem? Please describe.__

In a collaboration diagram I'd like to visualize the fact that the process process referenced via a call activity interacts with other activities / participants via message passing.

This is not allowed per BPMN spec. __It turns out, according to @falko's interpretation it is allowed by the spec, it is simply not correctly represented in UML _and_ the meta-model__.

It is not supported by our tools, as the call activity is not an interaction node:

__Describe the solution you'd like__

I'd like us to allow an exception to the BPMN spec and allow call activities to be sources / targets of message flows.

__Describe alternatives you've considered__

Stick to the status quo.

__Additional Context__

From the users point of view we already do something similar by allowing data stores to be modeled between participants (cf. https://github.com/camunda/camunda-modeler/issues/183).

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Related to SUPPORT-4858, SUPPORT-10808

Depends on https://github.com/bpmn-io/bpmn-js/issues/1467

[MaxTru]

Also requested by https://jira.camunda.com/browse/SUPPORT-10808

Notice that the spec explicitly allows this:

[barmac]

Fixed upstream via https://github.com/bpmn-io/bpmn-js/pull/1468

[nikku]

Closed via https://github.com/camunda/camunda-modeler/pull/2335.