[metadata] provide better abstraction for redis client

[Jeffwan]

🚀 Feature Description and Motivation

see discussion here https://github.com/vllm-project/aibrix/pull/1639#discussion_r2418152646

Let's build better abstraction instead of directly reference a redis client.

At the same time, storage has a redis implementation https://github.com/vllm-project/aibrix/blob/main/python/aibrix/aibrix/storage/redis.py. however, the metadata case is different from object interface. I personally still prefer to have different abstraction

Use Case

code quality and extensibility

Proposed Solution

No response