Co-authored-by: Eugene Yurtsev <eyurtsev@gmail.com>

Update the repo README to note Google provided integrations

Fixes mypy error:
langchain_postgres/chat_message_histories.py:344: error: Cannot override
writeable attribute with read-only property [override] Found 1 error in
1 file (checked 37 source files)

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Co-authored-by: Eugene Yurtsev <eyurtsev@gmail.com>

This PR includes
* New PGVectorStore classes (in namespace v2)
* Migration module and guide
* Deprecation warning for PGVector
* Updated documentation

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Co-authored-by: dishaprakash <57954147+dishaprakash@users.noreply.github.com>

#167)

Removed the definition of the duplicate index on the id column and
removed the column attributes that do not match what PostgreSQL uses for
a primary key. A primary key always has an index and is always not
nullable.

Co-authored-by: Eugene Yurtsev <eyurtsev@gmail.com>

I've decided to keep the Google Cloud Integrations section at the
bottom. I will send separate PRs for the migration script and quickstart
update.

Co-authored-by: Eugene Yurtsev <eyurtsev@gmail.com>

chore: Update PGVectorstore quickstart

Changes:
- The instances where the variable `engine` is being used instead of
`pg_engine`
- Added Cohere embedding model along with the right vector size for the
model
- Change to use `embedding` parameter instead of `embedding_service` in
vectorstore.afrom_documents

Fixes #180

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Co-authored-by: Averi Kitsch <akitsch@google.com>