Allow assertions on the messageKey

[jonathanlukas]

Description

Sometimes, an API call does not return the whole message event, but just a key.

Then, I would like to be able to perform message assertions based on this key in a nice way (like the InspectedProcessInstance)

[saig0]

Note

Heads up! We are building a new Java testing library for Camunda 8.6. The new library will replace Zeebe Process Test.
Read more about upcoming changes here and stay tuned for updates. 🚀