feat: added ability to register workers manually

[idbenami]

PR Checklist

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• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

When BullExplorer.OnModuleInit is finished the processors are starting to process jobs from the queue while my app isn't ready, and there's no way to tell the processors when they can start taking jobs.

Issue Number: 2030

What is the new behavior?

Added an option to manually register workers and event listeners using a provider called BullRegistrator. by default nothing has changed, but if manualRegistration is set to true the developer need to call BullRegistrator.register in order for the workers and event listeners to start working.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[idbenami]

@kamilmysliwiec any estimation on merging?

[ramigoldvarg]

Is there an estimate when this will be added? Waiting for this capability

[jarPotato]

Waiting for this to be merged. It should also help #2017 .