draft: Add an optional REST Controller for Celery

This adds a Flask-based remote control for Celery as an extra bootstep that is enabled by the Celery consumer blueprint (when the Celery worker is started), when the rest_controller flag is enabled in the Celery config.

This remote-control enables the same operations as the celery inspect and celery control protocols, as GET and POST endpoints respectively.

TODO:

• Improve thread management / exception passing (the Celery "shutdown" remote control just raises an exception, and waitress as WSGI server eats all exceptions; We could probably have a single WSGI thread, instead of a pair of WSGI "control" and WSGI "serving")
• basic unit testing

assigned to @olasd

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• 75327347 - Add an optional REST Controller for Celery

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mentioned in issue swh/infra/sysadm-environment#4839

mentioned in issue swh/infra/sysadm-environment#5023

added 60 commits

• 75327347...e3d9963e - 58 commits from branch swh/devel:master
• deaebf01 - celery_backend: set yaml Celery config at the proper level
• cf9a108f - Add an optional REST Controller for Celery

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