Add support for MongoDB Atlas $vectorSearch vector search

[baileympearson]

This PR is the JS equivalent of langchain-ai/langchain#11139.

This PR add support for the $vectorSearch stage in the MongoDB Atlas VectorStore. $vectorSearch was announced at MongoDB .local London.

Some notes:

• We don't have access to Cohere, so I changed the existing integration tests to use OpenAI. I'm happy to change them back if needed but I'll need help getting access to Cohere. The tests pass locally for me but I left them skipped because they're skipped in main.
• I made a few cleanups to the MongoDB atlas implementation and isolated them in this commit. Happy to revert.

Same as with the Python implementation, this is a breaking change.

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Name	Status	Preview	Updated (UTC)
langchainjs-docs	✅ Ready (Inspect)	Visit Preview	Oct 9, 2023 10:58pm

[dosubot]

This comment flags a change in the code that explicitly accesses an environment variable via process.env, and it should be reviewed by maintainers.

[jacoblee93]

Seems fine, is there no SDK update required?

[jacoblee93]

Could you also please run yarn format and yarn lint?

[baileympearson]

Hey @jacoblee93 - I linted and formatted, hopefully it passes CI now.

The changes here are compatible with the existing mongodb version specified - no dependency bump required.

[jacoblee93]

Thank you!

[jacoblee93]

@baileympearson I get this when running integration tests:

$vectorSearch is not allowed or the syntax is incorrect, see the Atlas documentation for more information

Do I need to upgrade something else? If so, can we add something to the docs with instructions?

[baileympearson]

@jacoblee93 You must be using MongoDB 7.0 or later in Atlas - I've added a note to the documentation about this

[jacoblee93]

No backcompat is a bummer :(

But thank you for the docs! Will merge with tonight's release.