Support connecting the Priskv server directly

[Jeffwan]

🚀 Feature Description and Motivation

for single server user, user still need to introduce redis as the proxy which is unnecessary and redundant.

• before: engine -> redis -> priskv
• after: engine -> priskv

benefits

1. Simplify the setup and user doesn't need to add additional redis and maintain the metadata management

Use Case

• sidecar mode
• single host mode

Proposed Solution

No response

[Jeffwan]

C client support direct mode, however, the Python binding only exposes the cluster client API which requires Redis.

 // pybind.cpp:
  #include "../cluster/client/client.h" //  Only includes cluster client here

  | Feature           | Direct Mode   | Cluster Mode             |
  |-------------------|----------------------|--------------------------|
  | Redis required    | No                   | Yes                      |
  | Connection target | Single PrisKV server | Redis meta server        |
  | Use case          | Sidecar or single-host | Distributed cluster      |
  | Automatic routing | No (single server)   | Yes (key-based sharding) |
  | Complexity        | Low                  | Higher                   |

We need to fix the C direct client & cluster client first and then come back to fix the python binding problem