Multi-index querying for API layer

[KennyZhang1]

Description

This PR adds support for local and global multi-index querying to the graphrag query API.

Related Issues

N/A

Proposed Changes

• Implemented two new functions in the graphrag query API layer for multi index global and local search
• Cleaned up graphrag API by moving helper functions to a separate util file
• Added support for passing in lists of vector stores via the config file

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

• This PR is a first step for integrating multi-index querying into the graphrag library. Once the config file changes and config loading methods are finalized, a separate PR will be opened for integrating multi-index querying at the CLI layer.