Add Neptune chains

[michaelnchin]

Initial port of NeptuneOpenCypherQAChain and NeptuneSparqlQAChain out of the langchain-community package.

This also adds the base langchain package as a new dependency.

To import, run:

from langchain_aws.chains import (
    NeptuneOpenCypherQAChain
    NeptuneSparqlQAChain
)

[3coins]

@efriis
Is there any concern on adding langchain as a direct dependency here?

[3coins]

@michaelnchin
Thanks for submitting this change. Adding a direct dependency on langchain will not only increase the size of the package, but also increase the dependency footprint of langchain-aws. We should pivot to updating the code to start using LCEL chains, so there is no direct dependency on langchain. Here is an example where LangChain moved their StuffDocumentsChain to a factory method that returns an LCEL chain, we should follow the same strategy.
https://github.com/langchain-ai/langchain/blob/1378ddfa5fa901179a26e0e2a91cbea52de625cf/libs/langchain/langchain/chains/combine_documents/stuff.py#L24