Add Volcengine LLM (doubao) support

[likeaturtle]

📝 Description

🗣️ Type of Change

• 🐞 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 📖 Documentation update
• ⚡ Code refactoring (no functional changes, no api changes)

🤖 AI Code Generation

• 🤖 Fully AI-generated (100% AI, 0% Human)
• 🛠️ Mostly AI-generated (AI draft, Human verified/modified)
• 👨‍💻 Mostly Human-written (Human lead, AI assisted or none)

📚 Technical Context (Skip for Docs)

• Reference: [URL]
• Reasoning:

1. Configuration structure update (config.go)
   1.1 Added the VolcEngine field to the ProvidersConfig struct
   1.2 Initialized the VolcEngine configuration in the DefaultConfig() function
2. Provider creation logic (http_provider.go)
   2.1 Added two types of support for VolcEngine: Explicit provider configuration: Supports provider: "volcengine" or provider: "doubao"
   2.2 Automatic model name recognition: Automatically uses VolcEngine when the model name contains "doubao", "volcengine", or starts with "doubao"
   2.3 Default API endpoint: https://ark.cn-beijing.volces.com/api/v3

🧪 Test Environment & Hardware

• Hardware: [NanoKVM Cube]
• OS: [NAME=Buildroot, VERSION=-g3649fe90d, ID=buildroot, VERSION_ID=2023.11.2, PRETTY_NAME="Buildroot 2023.11.2"]
• Model/Provider: [Volcengine]
• Channels: [Discord, Telegram, Feishu]

☑️ Checklist

• My code/docs follow the style of this project.
• I have performed a self-review of my own changes.
• I have updated the documentation accordingly.