feat(core): support optional dependencies in factory providers

[kamilmysliwiec]

PR Checklist

Please check if your PR fulfills the following requirements:

• 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?

Issue Number: N/A

What is the new behavior?

Introduces a new optional factory provider dependencies syntax:

providers: [
  {
    provide: 'FACTORY_PROVIDER',
    useFactory: optionalDependency => optionalDependency ?? defaultValue,
    inject: [{ token: 'RANDOM_OPTIONAL_DEP', optional: true }], // <- this is new 🔥 
  },
  { provide: 'RANDOM_OPTIONAL_DEP', useValue: 'OPTIONAL_DEP_VALUE' },
],

Does this PR introduce a breaking change?

• Yes
• No

Other information

[coveralls]

Pull Request Test Coverage Report for Build 833a9586-e3d4-4777-8dd0-27fb96ec52c4

• 20 of 20 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 94.172%

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Totals
Change from base Build 0c6ff061-43cb-4dba-bbee-5da440fdbcd6:	0.02%
Covered Lines:	5736
Relevant Lines:	6091

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💛 - Coveralls