Running rake commands directly via `command:` in recurring tasks configuration

[marelons1337]

Hi!
I am moving my workers from http://github.com/javan/whenever schedule to solid_queue and the command key in recurring tasks could be improved with rake tasks handling.
Here's a simple example based on my https://github.com/ankane/pghero setup

# config/schedule.rb
every 10.minutes do
  rake "pghero:capture_query_stats"
end

I believe this would be easy to implement by simply letting users do:

# config/schedule.yml
  capture_query_stats:
    command: "rake pghero:capture_query_stats"
    schedule: every 10.minutes

Currently it produces below error

undefined local variable or method 'capture_query_stats' for an instance of SolidQueue::RecurringJob

Is there something I am missing here and it's possible to execute the above? Right now we are using a simple workaround to get this to work:

# config/schedule.yml
  capture_query_stats:
    class: Rake::TaskJob
    args: "pghero:capture_query_stats"
    schedule: every 10 minutes
    queue: default

# app/jobs/rake/task_job.rb
class Rake::TaskJob < ApplicationJob
  queue_as :default
  self.queue_adapter = :solid_queue
  retry_on StandardError, attempts: 0

  def perform(command)
    require "rake"
    Rake.application.init
    Rake.application.load_rakefile
    Rake::Task[command].invoke
  end
end

But it does seem a bit ovet the top. I am happy to open a PR for this improvement.