Add FewShotExamples managed value

[nfcampos]

• (API will change, this is a draft)
• Add .score() method on checkpointer to mark checkpoints as "good"
• Add .list_w_score() method on checkpointer to list "good" checkpoints
• Implement score/list_w_score in sqlite and aiosqlite checkpointers
• Add FewShowExamples managed value which can be used to attach few shot examples to state object
• Add sync and async examples in tests

[hinthornw]

Should/are scores scoped to a node level? or whole graph? Any way to differentiate?

[andrewnguonly]

Closing this PR in favor of https://github.com/langchain-ai/langgraph/pull/444.