feat(model): add applyDefaults() helper that allows applying defaults to document or POJO

[vkarpov15]

Re: #11945

Summary

Model.applyDefaults(doc) applies the defaults specified in the model's schema to doc. doc can be either a document or a POJO/lean document.

Couple of use cases:

1. Create a document with defaults: false, because you want to apply defaults later after some other logic.
2. Making it easier to write plugins like mongoose-lean-defaults, etc.

Examples

[AbdelrahmanHafez]

This is awesome, thanks!

Is there a way where we can use applyDefaultToPOJO inside applyDefaults? I'm concerned that on the long run we'll add some rules to applyDefaults that won't be applied to applyDefaultsToPojo and they'll go out of sync. This won't happen if applyDefaults relies on applyDefaultsToPojo

What do you think? @vkarpov15

[vkarpov15]

@AbdelrahmanHafez I would love to make it so that we follow the same code path for both documents and POJOs. But unfortunately there's a lot of little edge cases. For example, for documents we need to set paths to default state, but not for POJOs. For POJOs, we need to recursively call applyDefaultsToPOJOs() for subdocs, but not for documents, because subdocuments call applyDefaults() recursively internally within the constructor. There probably is a way to unify the code paths without introducing any performance issues, but I'd like to get better benchmarking in place first.