Extract run behaviour into a concern

[jenshenny]

At Shopify, we have custom ApplicationRecord models that we would want Run to inherit from.

This PR extracts the behaviour of the Run model to a concern. This enables custom run models to include the behaviour from the Run model, while being able to inherit from different ApplicationRecord from the gem standard.

class CustomRun < CustomApplicationRecord
  include MaintenanceTasks::RunConcern
end

[etiennebarrie]

We already have an undocumented way to inject a Run model in Runner:

maintenance_tasks/app/models/maintenance_tasks/runner.rb

Line 42 in 799a022

def run(name:, csv_file: nil, arguments: {}, run_model: Run, metadata: nil)

We don't have a way to share the code with another class with a different superclass, that's true. Let's just make this a private API though.