feat: add comprehensive Rust implementation with feature parity

This commit completes the Rust implementation of mail2couch with full feature
parity to the Go version, including:

- Complete IMAP client with TLS support and retry logic
- Advanced email parsing with MIME multipart support using mail-parser
- Email attachment extraction and CouchDB storage
- Sync mode implementation with deleted message handling
- Enhanced error handling and retry mechanisms
- Identical command-line interface with bash completion
- Test configurations for both implementations

The Rust implementation now provides:
- Memory safety and type safety guarantees
- Modern async/await patterns with tokio/async-std
- Comprehensive error handling with anyhow/thiserror
- Structured logging and progress reporting
- Performance optimizations and retry logic

Test configurations created:
- rust/config-test-rust.json - Rust implementation test config
- go/config-test-go.json - Go implementation test config
- test-config-comparison.md - Detailed comparison documentation
- test-both-implementations.sh - Automated testing script

Both implementations can now be tested side-by-side with identical
configurations to validate feature parity and performance.

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

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

rust/Cargo.toml

@@ -36,6 +36,9 @@ async-std = { version = "1.12", features = ["attributes"] }
 # TLS support for secure IMAP connections
 async-native-tls = "0.5"
 
+# Email parsing with MIME support
+mail-parser = "0.6"
+
 # Logging
 log = "0.4"
 env_logger = "0.10"