fix: fix factory provider definition

[zanminkian]

According to this document, if factory provider's useFactory function is async, useFactory function should return Promise<T>.

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?

Does this PR introduce a breaking change?

• Yes
• No

Other information

[coveralls]

Pull Request Test Coverage Report for Build 5fc2f9c8-81c3-4daa-9162-8f82444480a5

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 94.13%

---

Totals
Change from base Build 6c6442b4-a131-4d7f-a0f8-d2eb1814def7:	0.0%
Covered Lines:	5693
Relevant Lines:	6048

---

💛 - Coveralls

[kamilmysliwiec]

This is a breaking change so we might need to postpone it till the next major release.

[micalevisk]

@kamilmysliwiec could you add the label status: blocked 🚫 to this PR