Add summarize the long-content feature

[cysummer]

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1. Is this request related to a challenge you're experiencing? Tell me about your story.

I'm trying to get the agent to summarize the document in knowledge base，contains long document.

2. Additional context or comments

langchain has already implemented this feature, and I hope Dify can also have this feature. https://python.langchain.com/v0.1/docs/use_cases/summarization/

3. Can you help us with this feature?

• [ ] I am interested in contributing to this feature.

[iguoli]

I really also need this feature! I'm using python code to chunk the long-content into several pieces and transfer them to an iteration to implement the feature for now, but the performance was not good and iteration was limited to 30 elements maximum.

[luizcarvalho]

I really also need this feature!!