feat: Make Celery workers and queues configurable

[webjunkie]

Description

We want to run separate Celery workers that consume a different set of queues each.
See RFC https://github.com/PostHog/product-internal/pull/537

Type of change

• New feature (non-breaking change which adds functionality)

Changes

• add possibility to configure several workers, then use range in the template
  • please advise if this is the way to go or not at all
• we can pass the config options as environment variables, which override Celery default settings
• by default, nothing should change right now
  • but allows that we can put a config into dev environment and then see how it works

Example config for dev later:

  workers:
    - name: priority
      queues:
        - email
    - name: default
      queues:
        - celery # default queue for Celery
        - insight_refresh
        - insight_export
    - name: gevent
      pool: gevent
      concurrency: 50
      queues:
        - analytics_queries

How has this been tested?

• how can I properly test this?

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works

[webjunkie]

@frankh Can you take a look here?