fix(openai): support custom headers in chat model config

[redflyingfish]

What type of PR is this?

fix

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• The description of this PR title is user-oriented and clear enough for others to understand.
• Attach the PR updating the user documentation if the current PR requires user awareness at the usage level. User docs repo

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fix(openai): 支持在 chat model config 中配置自定义请求头

(Optional) More detailed description for this PR(en: English/zh: Chinese).

en:
This PR adds CustomHeaders support to components/model/openai.ChatModelConfig.

Previously, OpenAI chat model users could pass custom headers through request options, while agenticopenai.ChatConfig already supported config-level custom headers. This PR aligns the OpenAI chat model config with that behavior by forwarding CustomHeaders to the underlying ACL OpenAI client.

It also updates the OpenAI component README files and adds a regression test.

Test:

cd components/model/openai
go test ./...

#### (Optional) Which issue(s) this PR fixes:

Fixes https://github.com/cloudwego/eino/issues/1048

#### (optional) The PR that updates user documentation:

N/A. Component README files in this PR were updated.

[mrh997]

Thank you for your PR~