Allow for injection of BSON library into MongoClient

Currently, the driver relies on the 1.x branch of BSON. It does some auto-detection to detect if bson-ext is present, and will use that instead.

In the next major version of the driver, we will be upgrading to use BSON 4.x.

Instead of using auto-detection logic like we do currently for bson-ext, we should allow users to provide their own BSON implementation to the MongoClient.

Default behavior:

// Uses BSON v4

const client = new MongoClient(url);

// Import bson-ext (will likely require a breaking version of bson-ext for this)

const bson = require('bson-ext');

// MongoClient now uses bson-ext

const client = new MongoClient(url, { bson });