When working with other serialization formats, like protobuf, it would be nice to have direct access to ObjectID's internal buffer property.

As seen in this code line (https://github.com/mongodb/js-bson/blob/1576fd0bc454b869b6445ac58e27810a7a221cb3/src/objectid.ts#L49 ), property/symbol `id` is marked as `private`.

This implies in conversion from hex string after calling the method `toHexString`, unnecessarily:

let buffer = Buffer.from(objectId.toHexString(), 'hex')

So, my proposal is to remove the `private` modifier of `id`property, or to implement a method `toBuffer`.

I can develop it and make a PR if you think it is nice to have.