Implement MongoDB Checkpointer

[hegeduscs]

Add the option to use MongoDB or compatible NoSQL databases as Checkpointer to Langgraph.

[hinthornw]

We aren't adding integrations directly in this package, though we plan to add a how-to for implementing a checkpointer

[ssoto]

Are you thinking on add a langgraph-community where the community add custom imlementations?

[vbarda]

@ssoto not currently. we do provide some reference implementations here: https://langchain-ai.github.io/langgraph/how-tos/persistence_postgres/. also, someone from community just implemented one for MongoDB, might be useful for you #943