refactor: sets correct typing to inject property

[ivanzigoni]

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?

The inject property at HttpModuleAsyncOptions was typed as any[], causing some confusion.

Issue Number: N/A

What is the new behavior?

The inject property at HttpModuleAsyncOptions is now typed as InjectionToken that was brought from nest core repository.

Does this PR introduce a breaking change?

• Yes
• No

Other information

I would also like to suggest adding a section to the module's documentation page about injecting standalone providers since only registering them at extraProviders is not enough. As soon and if this type refactor is merged I will myself open another pr updating the docs.