feat: pathsToSave option to save() function

[IslandRhythms]

closes #9583

[FFUV]

LGTM!

[vkarpov15]

Let's also add some more test cases that cover:

1. Single nested subdocs. What happens if I do save({ pathsToSave: ['subdoc.name'] }) and save({ pathsToSave: ['subdoc'] }) where we have subdoc: Schema({ name: String })? Make sure to cover both cases where name is modified like doc.subdoc.name = 'foo' as well as doc.subdoc = { name: 'bar' }
2. Doc arrays using the same cases as above