GraphRAG: refine config usage and fix some bug.

[SonglinLyu]

Description

1. Use config as default instead of environment parameters.
2. Fix some fmt.
3. Merge enable_summary and graph_community_summary_enabled config.
4. add GraphRAG config in configs/dbgpt-graphrag.toml.

Fix #2312

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

Snapshots:

Include snapshots for easier review.

Checklist:

• My code follows the style guidelines of this project
• I have already rebased the commits and make the commit message conform to the project standard.
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• Any dependent changes have been merged and published in downstream modules

[Aries-ckt]

LGTM

[fangyinc]

LGTM.