Add Support for Couchbase Checkpointer library

[lokeshgoel177]

Hi Team,
Greetings from Couchbase

We at couchbase would like to add Couchbase as a Checkpointer DB in Langgraph. We have initial implementation in the form of Python Notebook here: #1576
Please let us know what all will be required to add a library just like langgraph_checkpoint_postgres.

Thanks