Add GenericEnqueuer for consistent job priorities

[johha]

Some delayed jobs enqueue other delayed jobs. For example, an app delete job may enqueue secondary jobs like blobstore deletion and buildpack cache cleanup.
If CAPI is configured with dedicated job priorities or dynamic job priorities is enabled, these secondary jobs might unintentionally receive a higher priority than their primary job. This could lead to less critical jobs, like blobstore deletion, being processed before more critical ones, such as service instance creation.

This change introduces GenericEnqueuer, a singleton enqueuer ensuring that all jobs enqueued within the same job execution context inherit the same priority.
It is automatically initialized and destroyed within the CCJob wrapper, ensuring consistent priority propagation. Jobs can now be enqueued using:
GenericEnqueuer.shared.enqueue(job)

• Priority Incrementation:
  • enqueue and enqueue_pollable now support an optional priority_increment parameter.
  • This increases the priority value (making the job less important), allowing secondary jobs (e.g., blobstore deletion) to have a lower priority than their parent job.
• Preserving Priority for Re-enqueued Jobs:
  • The preserve_priority flag ensures that re-enqueued jobs retain their previous priority.
  • Prevents unnecessary priority increases on repeated executions.
• Config Priority Handling:
  • If a job has a dedicated priority configured, it is added to the current priority and priority_increment, ensuring correct propagation.

Job Type	Preserve Priority	Config Priority	Increment	Final Priority
Parent Job	❌ No	100	nil	100
Sub-Job A	❌ No	200	50	350 (100+200+50)
Sub-Job B	❌ No	nil	50	150 (100+50)
Sub-Job C	❌ No	nil	nil	100 (inherits parent)
Tertiary Job A1	❌ No	50	20	420 (350+50+20)
Tertiary Job B1	✅ Yes	10 (ignored)	30	150 (preserved from Sub-Job B)
Tertiary Job C1	❌ No	nil	50	150 (100+50)
Re-enqueued Job	✅ Yes	20 (ignored)	100	42 (original preserved priority)

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