Stunner - Missing the ability to to specify the FEEL expressions

Missing the ability to to specify the FEEL expression in the process designer. It's actually possible in the legacy process designer, so the goal for this task is about reaching feature parity and being able to specify the expressions in the same way.
It's expected to be present for the sequence flow's branches.

Tasks:

• Add the expression type in the dropdown
• Only applies to SeqFlow?
• Validate the input? (server/client)
• Any other implication on the script side?
• Marshalling
• Testing