Extract component for sourcing from events

[mostlyobvious]

I see 3 areas that are interested in sourcing events in the RES ecosystem:

• AggregateRoot, obviously
• RES::Projection
• event-sourced process managers like in https://blog.arkency.com/process-managers-revisited/

At the moment all three sort-of reimplement sourcing part and I believe this area could be improved.