feat: add prefix cache support for volcengine

[laoshancun]

There are 2 api change.

• cache context create

POST http://oneapi/v1/context/create
Authorization: Bearer token
Content-Type: application/json
 
{ 
    "model":"doubao-pro-32k", 
    "messages":[ 
        {"role":"system","content":"long prompt need to cache"}
     ], 
     "ttl":86400, 
     "mode": "common_prefix"
}

• use cache context

POST http://oneapi/v1/chat/completions
Authorization: Bearer token
Content-Type: application/json
 
{
    "context_id": "ctx-20250218162710-6jwbk", // context_id from context create api result
    "model": "doubao-pro-32k",
    "messages":[ 
       {"role":"user","content":"hello!"}
     ]
}

see also：https://www.volcengine.com/docs/82379/1396490