Add metadata filter support for external knowledge base

[moonheart]

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

We use Milvus as external knowledge base, and need to use "metadata filter" to implement some features.

Currently, only builtin knowledge base support metadata.

2. Additional context or comments

Builtin knowledge base contains metadata settings:

Which is missing in external knowledge base:

3. Can you help us with this feature?

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

[macchen123]

+1, ragflow also needs this

[liqilong2017]

+1，dify+milvus，need this

[elepherai]

+1, ragflow also needs this

[hbprotoss]

when will it be released?

[qingyumonian]

when will it be released?