Add reworked Neptune chains

[michaelnchin]

Replaces #306.

Adding new LCEL format chains create_neptune_opencypher_qa_chain and create_neptune_sparql_qa_chain for Amazon Neptune.

These will replace the legacy NeptuneOpenCypherQAChain and NeptuneSparqlQAChain chains in langchain-community.

To import, run:

from langchain_aws.chains import (
    create_neptune_opencypher_qa_chain,
    create_neptune_sparql_qa_chain,
)

[3coins]

@michaelnchin
This looks great! One minor comment, looks good otherwise.

Another thing to note is that users will benefit from a sample notebook with examples for using message history with these new LCEL chains, as the implementation is very different from the legacy memory classes. These notebooks also need an update, once we add deprecation messages to the LangChain repo.
https://github.com/langchain-ai/langchain/blob/c55af44711ba9180ce8a51a55a385f31023341b5/docs/docs/integrations/graphs/amazon_neptune_open_cypher.ipynb

[michaelnchin]

Another thing to note is that users will benefit from a sample notebook with examples for using message history with these new LCEL chains, as the implementation is very different from the legacy memory classes.

Added in langchain-ai/langchain#29260.