ref(processor): Implement a new structure for the processor

[iambriccardo]

This PR introduces a new processor structure, currently implemented for check-ins only.

The new structure has the following goals:

• Each processing group is defined in its own file. The file should contain the processing logic and all the methods specific to that group.
• Shared processing functions should be placed in the common folder inside groups.
• The state of each group is built from a common set of parameters, with the state clearly defined for each processing group. This makes it easy to understand the functionality of each group.
• All processing functions now use a GroupPayload abstraction, which abstracts away whether the function operates on the ManagedEnvelope and the Event or only the ManagedEnvelope.
• Dispatch of processing is done statically via a macro, which requires pairs of ProcessingGroup -> ProcessGroup.
• The processor will have less and less code that will be moved into each individual group.

Upcoming work:

• Continue porting processing groups to the new structure and move the group specific functions into their own group within groups.
• Move ProcessingGroup into the groups module in addition to other structs.
• Refine the abstractions further, if necessary.

[iambriccardo]

Temporarily duplicated, to make it work for both old and new processing mechanism.

[jjbayer]

I feel like it should be possible to do this dispatch directly with dynamic trait objects instead of an enum + macro.