Add upserting capability to PGVector

[angelol]

Currently, the PGVector vectorstore only supports adding embeddings to the store. This PR adds upserting capability to the store, so you can update existing embeddings if the underlying text have changed. This is useful if you have indexed a set of documents but some of the documents are being changed and you want to keep the index up-to-date.

To use this feature, simply provided ids for all texts/or documents. The pgvector notebook has been updated accordingly.

The lack of upserting capability has been mention in issue #6866.

👀 @rlancemartin, @eyurtsev

In case you want to announce on Twitter, my Twitter handle is @angelol

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Name	Status	Preview	Comments	Updated (UTC)
langchain	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jul 13, 2023 9:53am

[leo-gan]

@angelol Hi , could you, please, resolve the merging issues? After that ping me and I push this PR for the review. Thanks!

[efriis]

Closing because the PR wouldn't line up with the current directory structure of the library (would need to be in /libs/langchain/langchain instead of /langchain). Feel free to reopen against the current head if it's still relevant!