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

[Reference any related issues or tasks that this pull request addresses.]

Proposed Changes

[List the specific changes made in this pull request.]

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).

Additional Notes

[Add any additional notes or context that may be helpful for the reviewer(s).]

[natoverse]

This is implemented as part of config updated in v3. See migration notebook for examples: https://github.com/microsoft/graphrag/blob/main/docs/examples_notebooks/index_migration_to_v3.ipynb