[Request] Implementation of "useExisting" provider

[jbjhjm]

Feature Request

I need to provide a service both directly and via a provider token.
Both need to inject the same instance.
For this use case, angular implements a ExistingProvider.

Currently this is only possible by implementing a manual singleton pattern through a factory function.

It would be awesome to see the ExistingProvider in nestjs too and would further reduce differences to angular.

Thanks.

[kamilmysliwiec]

Published as 6.3.0

[jbjhjm]

Works perfectly fine. Thank you Kamil!!

[lock]

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