feat(NODE-7125): expose db and client

[durran]

Description

Exposes db and client in the public API.

What is changing?

• collection.db exposes the Db object
• db.client is now public and exposes the client.

Is there new documentation needed for these changes?

None

What is the motivation for this change?

NODE-7125

Release Highlight

Collection and Db objects now provide references to their Db and MongoClient

import { MongoClient } from 'mongodb';

const client = new MongoClient(process.env.MONGODB_URI);
const db = client.db('test');
assert(db.client === client); // returns the MongoClient associated with the Db object
const collection = db.collection('test');
assert(collection.db === db); // returns the Db associated with the Collection object

Double check the following

• Ran npm run check:lint script
• Self-review completed using the steps outlined here
• PR title follows the correct format: type(NODE-xxxx)[!]: description
  • Example: feat(NODE-1234)!: rewriting everything in coffeescript
• Changes are covered by tests
• New TODOs have a related JIRA ticket