Add extraProviders also to forRootAsync

[SmallhillCZ]

Is there an existing issue that is already proposing this?

• I have searched the existing issues

Is your feature request related to a problem? Please describe it

It's the same issue as #495, but for forRootAsync()

Describe the solution you'd like

I tried to implement it in datageo-cz/bull@afd6f5c, but I end up with following error, which is there even if I revert my changes, so it's probably unrelated, but can't test if it works.

Error: Nest can't resolve dependencies of the BullExplorer (?, DiscoveryService, BullMetadataAccessor, MetadataScanner). Please make sure that the argument ModuleRef at index [0] is available in the BullModule context.

Teachability, documentation, adoption, migration strategy

No response

What is the motivation / use case for changing the behavior?

Using global config from injected Config in the global forRootAsync()

[kamilmysliwiec]

Would you like to create a PR for this issue?