Add ability to set a custom runtime lock key for :until_and_while_executing strategy

[sharshenov]

This PR adds ability to set a custom runtime_lock_key for :until_and_while_executing strategy

class MyJob < ApplicationJob
  unique :until_and_while_executing

  def perform(args)
    # rocket_science(args)
  end

  def runtime_lock_key
    'my_lock_key_for_job_locking_while_it_is_being_performed'
  end
end