Feature/#166 create element templates for workers

[lmoesle]

closes #166

The main goal is to create C7 element templates for workers with a maven plugin. I also prepared the maven module structure of the plugin to be adaptable to add more process engines and a gradle plugin in the future if the usecases arise.

[zambrovski]

@lmoesle I believe you forgot to check-in some classes. In addition, I would like to create an SPI so the generator doesn't have a direct dependency to a concrete engine. Instead the engine-specific settings can be provided as a dependency to the maven plugin. Since we already have the engine specific adapter, it could be an artifact of that...

But first things first - please add c7/c8 templates and configuration and then let us work on separation.

[lmoesle]

The tests ExternalTaskCompletionAfterCommitTest, ExternalTaskCompletionBeforeCommitTest and ExternalTaskCompletionWithoutTransactionTest are flaky and caused the pipeline to fail due to timeouts in the tests. I fixed the pipeline by rerunning it 😅

[zambrovski]

I understand what you are aiming and understand how you built it. Thank you for contribution!

Along with some minor issues like missing docs and warnings produced by the plugin during the build, I still believe the engine-specific code should not be in the worker.

But I'll need some time to understand it in more details before I can propose a change. Is it ok for you?

[lmoesle]

@zambrovski I did some refactoring and added camunda 8 element template generation too. I decided to create maven plugins for each engine for now. My PR is a kind of a prototype for a similar feature we already implemented in miranum.

Please take your time and give me feedback and suggestions. We can leave the PR for now as it isn't time critical