docs: Add comprehensive documentation for HTTP client authentication

Add detailed usage manual and principles guide for @requestauth annotation system,
covering all authentication types, usage scenarios, design principles, and internals.

**Documentation Structure:**

1. **HTTP_CLIENT_AUTH_USAGE.md** - User-facing usage manual
   - Quick start guide
   - Authentication types (Bearer, Basic, API Key, Custom Provider)
   - Usage scenarios (interface/method/parameter levels)
   - Best practices (security, configuration, Provider patterns)
   - Common Q&A
   - Complete examples with HTTP request formats

2. **HTTP_CLIENT_AUTH_PRINCIPLES.md** - Internal principles guide
   - Architecture overview with component diagrams
   - Core components (AnnotationParser, StaticAuthApplier, AuthFieldMapper, etc.)
   - Detailed workflow with execution traces
   - Key design decisions (constructor injection, name attribute overloading)
   - Consistency with FEL Tool system
   - Extension guide for custom authentication types

**Content Highlights:**

- 📖 80+ code examples covering all scenarios
- 🎯 5 detailed execution flow diagrams
- 📊 Component responsibility matrix
- 🔧 Extension patterns for custom authentication
- ⚖️ Design decision rationales
- ✅ Best practices and anti-patterns

**Coverage:**

- All 4 authentication types: Bearer, Basic, API Key, Custom
- 3-level configuration: Interface, Method, Parameter
- Static vs dynamic (Provider) authentication
- Field update mechanism for parameter-level auth
- BASIC auth dual-parameter support with name attribute
- Integration with BeanContainer and FEL Tool system

**Target Audience:**

- Usage Manual: Developers using HTTP client authentication
- Principles Guide: Framework developers and contributors

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: CodeCasterX