Add CampaignDeliverySummary module for aggregating delivery data

[neal-bpm]

#476

Describe your changes

This module provides a data structure to consolidate campaign delivery information into a summary format suitable for various use cases.

• Track total and completed task counts
• Group assigned deliveries by rider name
• Collect unassigned deliveries separately
• Implement Collectable protocol for use with Enum.into/2
• Include delivery details: dropoff name, status, and item names

Checklist before requesting a review

• I have performed a self-review of my code
• If it is a core feature, I have added tests.
• Are there other PRs or Issues that I should link to here?
• Will this be part of a product update? If yes, please write one phrase
  about this update in the description above.

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[mveytsman]

I haven't seen this before what does it do?

[neal-bpm]

By implementing Collectable for CampaignDeliverySummary, we can iterate using

summary = Enum.into(tasks, CampaignDeliverySummary.new(campaign))

Here initial state = CampaignDeliverySummary.new(campaign)

Similar to Enum.reduce pattern

defimpl Collectable do
    defp collect(cds, {:cont, task}), do: @for.add_task(cds, task)
    defp collect(cds, :done), do: cds
    defp collect(_cds, :halt), do: :ok
    def into(tasks), do: {tasks, &collect/2}
 end

Since we are collecting for the same module, we use @for.

def add_task(cds, task) do
    cds
    |> Map.update!(:total, &(&1 + 1))
    |> completed(task)
    |> delivery(task)
  end

Reference: https://hexdocs.pm/elixir/1.19.5/Collectable.html
Is this make sense?