Fix: Table 'langchain_pg_collection' is already defined for this MetaData instance

[0xrushi]

Possible solution for #165

Added thread locks to prevent multi table creation, optionally we can also pass extend_existing = True if required.

vector_store = PGVector(
    embeddings=OpenAIEmbeddings(),
    collection_name="test_collection",
    connection=connection_string,
    extend_existing=True,
)

[MartinGotelli]

This can be a private variable self._extend_existing

[t-ionut]

@MartinGotelli I've just encountered this exact issue in my work and I'm happy to see @0xrushi has a fix in this PR.

Any chance of getting this merged soon?