Administrative User Creation when using Docker image and removal of camunda-invoice - New Plugin

@camunda to date we have always loaded the initial administrative user manually or through a third party automation.

There does not appear to be any CLI or configs to load the initial administrative user when using a Shared Engine Deployment + the docker container

Typically when you load a blank deployment, when you enter the Camunda url you get something like:

As a result we threw together a proof of concept plugin based on the camunda-inovice generate user code.

This engine plugin generates an administrative user with admin permissions (same setup as the camunda-invoice admin) on engine startup. its configurable as well. See the README.

The code was quickly put together so there is cleanup and best practices to implement, but its functional and has basic error handling and logging.

feedback is more than welcome, and if we missed a undocumented way to load the intial administrative user, please let me know!!