feat(NODE-7009): add client metadata on demand

[durran]

Description

Allows appending driver info for the client metadata after a mongo client has been constructed.

What is changing?

• Adds appendMetadata to MongoClient
• Adds the ability for certain metadata to be modified on the client instance.
• Allows for appending metadata multiple times for the handshake.
• Syncs unified tests allowing for new functionality.
• Updates prose handshake tests.

Is there new documentation needed for these changes?

Manual docs.

What is the motivation for this change?

NODE-7009

Release Highlight

New appendMetadata API allows clients to add handshake metadata post construction

Driver information such as name, version, and platform are allowed:

import { MongoClient } from 'mongodb';

const client = new MongoClient(process.env.MONGODB_URI);
client.appendMetadata({ name: 'my library', version: '1.0', platform: 'NodeJS' });

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

[dariakp]

just a couple of small things left (also see comment on open thread)