[Proposal] Add support for MongoDB.Bson.ObjectId

[auribuo]

It would be really nice to have the option to use ObjectId as a backing type, because MongoDb relies a lot on it and it would remove some convertion that otherwise must happen.

[larsbloch]

I second this !

[andrewlock]

For feature requests like this, I think I've found an answer in the big redesign of the library in this PR:

• Fundamental redesign to allow complete customisation #117

The main idea is to make the library much more maintainable while also giving people a mechanism to customise the generated IDs as much as they like. This will make it easy to make changes like this 🙂