Define called element statically or as expression

[saig0]

Is your feature request related to a problem? Please describe.
For a call activity, I can define the processId of the called element statically as a string or dynamically as an expression.

Describe the solution you'd like

• the processId of the called element is defined in one attribute in the BPMN XML
• the attribute contains either a static value (e.g. processId) or an expression (e.g. ${processId})
• the expression is a valid FEEL expression
• the expression is evaluated when the element is activated
• the expression can access the variable context of the element
• if the evaluation fails (e.g. no variable with the given name) or the result is not a string then an incident is raised
• if the expression is not valid the deployment is rejected

Describe alternatives you've considered

• different attributes for a static value and an expression -> more effort when applying to more attributes since it is not a unify type/structure
• additional attribute to mark a value as an expression -> not possible if an element contains multiple attributes (e.g. task definition)
• additional attribute/element to override a static value with an expression -> too complex

Additional context

• the described solution for the call activity is also a blueprint for the other attributes
• the expression support for the processId was already implemented and will be changed to match the requirements