Allow user to provide customized collection_name

[AICrafterZheng]

Description

By default, the index name in GraphRAG is constructed using below code:

graphrag/graphrag/config/embeddings.py

Line 48 in fdb7e38

return f"{container_name}-{embedding_name}".replace(".", "-")

However, in some cases, users may want to connect GraphRAG to an existing embedding index rather than having GraphRAG generate a new one automatically.
This PR adds support for specifying a custom index name, enabling seamless integration with existing embedding stores.

Note: this is a reopened pr. Please see details in the closed pr #2016

Related Issues

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Proposed Changes

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Checklist

• I have tested these changes locally.
• I have reviewed the code changes.
• I have updated the documentation (if necessary).
• I have added appropriate unit tests (if applicable).

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