[FEATURE] allow create AND edit of dataExtension associated with event via event.schema

[JoernBerkefeld]

by default, one CAN create dataExtensions using schemas set within a eventDefintions simply by creating the event with a schema configuration in its payload. The event API does the rest for your.

However, once the eventDefinition is created it does not allow you to update the associated DE nor does it update the schema section based on changes to the associated DE. That leads to inconsistency and could be circumvented by running the update via a second call in mcdev, and similarly fetching the current fields from the DE during retrieve.

pre-requisites:

• [BUG] creating events fails if deKey is set alongside schema  #1406 / Bug/1406 creating events fails if dekey is set alongside schema #1417
• [FEATURE] test new rest endpoint for dataextensions #1361

FYI @DougMidgley