feat: allow the user to customize the http client

[xgfone]

Feature

1. Allow the user to customize http.Client, not the fixed &http.Client{Timeout: timeout}.
   If not configuring the http client, use http.DefaultClient instead.
2. For the original timeout config, use the timeout context context.WithTimeout to support it.

Motivation

The user maybe configure the proxy by the user-defined http.Client.

[Vein05]

Thanks for the pr, I will be reviewing it asap.

[Vein05]

Just letting you know, some tests are failing on this pr. Did you check them on your local development environment? Please read about how to test our package on https://github.com/cohesion-org/deepseek-go?tab=readme-ov-file#running-tests-1

[xgfone]

An error has been fixed. I had executed the command go test -v ./..., and it's ok.

[Vein05]

Thank you.

To answer your question on if we need to export those functions is yes as they are used by client and other api related functions.