`CREATE EXTENSION` fails with Azure Cosmos DB for PostgreSQL

[Joshua Sundance Bailey]

The SQL for enabling the vector extension is hardcoded at

langchain-postgres/langchain_postgres/vectorstores.py

Line 344 in 0705306

 "CREATE EXTENSION IF NOT EXISTS vector;" 

But when using Azure Cosmos DB for PostgreSQL, it's necessary to use SELECT create_extension('vector');

From relevant docs:

If CREATE EXTENSION fails with a permission denied error, try the create_extension() function instead. For instance:

SELECT create_extension('postgis');

To remove an extension installed this way, use drop_extension().

The same applies to pgvector in the same context.

I might propose a parameter that allows the user to define the create extension statement, or choose from a list of predefined options if that approach seems better. Currently, of course, the user can create the extension first and then use create_extension=False.

[Eugene Yurtsev]

Feel free to open a PR to try to parameterize