cleanup config and move config loading to connection init method

[whhe]

What problem does this PR solve?

Now the database is started at api/settings.py, but the configuration loading is in rag/settings.py, so there are two branch judgment logic. In this PR, I moved the configuration loading step to the init method of each connection class. In addition, the default table name of opendal is inconsistent in the code and service_conf.yaml, which I also fixed in this PR.

Type of change

• [ ] Bug Fix (non-breaking change which fixes an issue)
• [ ] New Feature (non-breaking change which adds functionality)
• [ ] Documentation Update
• [x] Refactoring
• [ ] Performance Improvement
• [ ] Other (please describe):

[whhe]

@KevinHuSh PTAL

[KevinHuSh]

Appreciations! By our evaluations, this feature is not going to be merged into main branch.