Add SolidQueue.recurring_task_job_class, accepting a class name

[ngan]

Customizing the job class that runs command recurring tasks currently looks like this:

Rails.application.config.after_initialize do # or to_prepare
  SolidQueue::RecurringTask.default_job_class = MyRecurringCommandJob
end

This has two downsides:

• It loads MyRecurringCommandJob at boot, along with SolidQueue::RecurringTask (and Active Record with it).
• SolidQueue::RecurringTask lives in the engine's app/models, so it's reloadable. A value set in after_initialize is lost on the first reload; it has to be set in to_prepare to stick.

This adds a SolidQueue.recurring_task_job_class setting that takes the job class name:

# config/application.rb
config.solid_queue.recurring_task_job_class = "MyRecurringCommandJob"

• It lives on the SolidQueue module, like the other settings, so it can be set through config.solid_queue and survives reloads.
• The name is resolved when a task is first enqueued, so the job class isn't loaded at boot. A class is still accepted too.
• It defaults to "SolidQueue::RecurringJob".

SolidQueue::RecurringTask.default_job_class (reader and writer) still works and delegates to the new setting, with a deprecation warning.