Add an interface in fileconsumer instead

[VihasMakwana]

Description: Create a Manager Interface for fileconsumer.
This would result in a cleaner and more readable code for threadpool PR.
Going with the interface is much better and it will keep the threadpool wholly isolated.

@djaglowski, I have the threadpool changes ready to be pushed. I need to know your POV for my approach to moving towards interfaces.

[VihasMakwana]

@djaglowski
If we keep only one Manager for both threadpool and the normal version for fileconsumer, it would be a mess with multiple if-else blocks everywhere in the code. Old code will be affected as well. It's better to create another Manager for threadpool which implements the ManagerInterface and keep it in a separate file.

[VihasMakwana]

@atoulme just to give you some context around the changes:

• This is regarding my ongoing work for [receiver/filelog] It doesn't respect poll_interval when files are too large #17846.
• After some discussion with @djaglowski, we have decided to go ahead with creating a featuregate for the thread pool approach. It makes much more sense to introduce an interface, instead of using a struct directly. This would further help in much more readable and user-friendly code for my final PR.

[djaglowski]

Will this result in duplicated code between two different implementations of the manager?

Ultimately, I don't think I can evaluate this change independently of how it is used.

[VihasMakwana]

Not needed