Add Optional Version Tag Property to Forms

[philippfromme]

• add versionTag property to forms
• versionTag property can be edited trough properties panel

As this property is required only in the context of the Camunda product stack we need to think about whether this should be added to the core or as an extension.

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Related to camunda/camunda-modeler#2295.

[nikku]

Removing this from F3 as it is out of scope, for now.

[nikku]

@philippfromme Consider to close this, we'll not support this for C7, and we just shipped support for C8 via #1243.