feat: more precise control over the timeout for requests to the LLM service

[LiteSun]

Description

Currently, AI-related plugins only have a timeout parameter. After reviewing the code, I found that timeout is the total of connect, read, and send timeout durations, making it impossible to precisely control the timeout for requests to the LLM service. This needs to be improved.

Currently https://github.com/apache/apisix/blob/master/apisix/plugins/ai-drivers/openai-base.lua#L70

...
local http = require("resty.http")
...

httpc:set_timeout(conf.timeout)

Expected:

...
local http = require("resty.http")
...

httpc:set_timeouts(connect_timeout, send_timeout, read_timeout)