Improve and extend processengine in apps

### Description

This epic is still a work in progress

The number of issues opened related to the process suggests there is a need to have a closer look at how we can improve the processes logic.
Creating an epic to keep track of all the Issues (feature/analysis) related to process.

The goal of this epic are:
- all optional tasks/steps in the process are defined in the BPMN
- extending the process is easy
- our code defining steps in the process should leverage the same interfaces as custom/external extensions to the process uses
- add support for the basic bpmn artifacts needed by service owners
- our bpmn definition should comply to the bpmn2 standard

As we discover more we should define the scope of this epic

### Up for consideration

This is a work in progress and there is still no guarantee that this will hold true!

- [x] Support coded gateways. When a gateway is defined in BPMN it should be possible to define code in the application to make the decision based on instance data. Long term solution for https://github.com/Altinn/app-lib-dotnet/issues/43
- [ ] Support service tasks. When a service task is defined in BPMN developers should be able to model what piece of code that should be executed. Long term solution for https://github.com/Altinn/app-lib-dotnet/issues/44

### In scope

TODO

### Out of scope

TODO

### Additional Information

_No response_

### Tasks

Tasks should be represented in smaller issues connected to this epic

No files, tests, or entry points are identified in this work-in-progress epic. Start by reviewing the related process issues, then define smaller issues for the listed BPMN goals; the epic is done when its scope is defined and those tasks are tracked separately.

Written by the indexing model from the issue text.