Add Volcengine provider configuration support

[Copilot]

Volcengine templates are now available in redc-template. The engine needs corresponding AKSK configuration support.

Changes

• mod/config.go: Added Volcengine provider struct with VOLCENGINE_ACCESS_KEY, VOLCENGINE_SECRET_KEY, and region fields following existing provider patterns
• README.md / README_CN.md: Added Volcengine configuration examples and environment variable documentation
• .gitignore: Added red-cloud binary exclusion

Configuration

providers:
  volcengine:
    VOLCENGINE_ACCESS_KEY: "AKIDXXXXXXXXXXXXXXXX"
    VOLCENGINE_SECRET_KEY: "WWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW"
    region: "cn-beijing"

Environment variables VOLCENGINE_ACCESS_KEY and VOLCENGINE_SECRET_KEY are automatically bound when config is loaded.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[需求] 配置文件需要支持火山引擎</issue_title>
<issue_description>目前模版库已提交火山引擎相关场景
https://github.com/wgpsec/redc-template/pull/15/files
https://github.com/wgpsec/redc-template/blob/copilot/implement-ecs-scene-volcengine/volcengine/ecs/main.tf

在 redc 引擎配置文件中也需要支持火山引擎的 aksk 配置</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [需求] 配置文件需要支持火山引擎 #15

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