Implement pattern to handle logic for user initiated process actions

[tjololo]

Description

When a user (person or system) performs actions like sign or reject we need to run some logic like signing the associated dataelements.
To make it possible to create custom logic that will run when a custom process action is performed we should leverage the same pattern to run our default logic.

Additional Information

This is initiated by Altinn/altinn-storage#207

Tasks

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Acceptance Criterias

No response