Use a class method to declare a CSV task

[etiennebarrie]

In #302 we went with a new subclass, which we refactored to a module to be included in a Task.

We got feedback in #302 (comment) but we can't really change the behavior only based on data from the Run (a CSV file being present), since we need to know something about the class to show the input which results in a CSV file being updated. The class itself needs to somehow declare that it supports CSV.

Rafael pointed out that we shouldn't use include as an API for changing behavior to the classes, we discussed that on Slack but we missed the mark with #313 when we went instead with a superclass, as explained in #310 (comment).

So instead this PR uses a class method on Task, doesn't require the Task authors to include a module or inherit from a different class. They simply call csv_collection. I'm open to feedback about the name, but the idea with this one is that it defines a collection like a Task author would do for you, so there some parallel between defining a method collection and calling csv_collection on the class.

Using a class method opens up features like being able to choose whether the CSV should have headers, changing the column separator, using converters, etc. as arguments to the method call that could be passed in to CSV.new.

Internally this still includes a module into the class rather than adding conditionals to collection and count and adding the csv_content attribute to the receiver.

[etiennebarrie]

One last thing I'm not sure about is whether to make the method private or not. On one hand it doesn't make sense to be called from outside the class, on the other hand things like attr_reader/belongs_to/before_action are not private and it's fine.

[adrianna-chang-shopify]

One last thing I'm not sure about is whether to make the method private or not.

I would be inclined to keep it public, given that most class-level macros are given public visibility in Rails.