Validate subscription correlation key for FEEL expression

[MaxTru]

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

The Zeebe Engine requires the subscription correlation key for a <bpmn:messages> to be a valid Feel expression. Otherwise the engine will throw an error.

Also see #272

Describe the solution you'd like

A validation ensuring that a Feel expression is entered for the subscription correlation key field of a bpmn:message is added. This can be done by checking for a leading = followed by a character sequence. Potentially in a second step this validation might use a more advanced Feel expression parsing.

Describe alternatives you've considered

See discussion in #272

Additional context

#272