Provide helper functions to compute the actual field label

[douglasbouttell-camunda]

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

In order to programatically access field labels outside of form-js, one has to use the templating helpers and reconstruct the internal execution state of the form.

For example, if someone wishes to add custom messaging around form validation. They will need to do this to display the correct field labels.

Describe the solution you'd like

Add a new method to the field object, computedLabel, which will compute the actual label to be shown.

This should centralise some logic which is currently spread over react code.

Describe alternatives you've considered

Labels can be properly generated but requires knowledge of the internal state of the form and access to helper functions.

Additional context

https://github.com/camunda/tasklist/pull/4515