Implement repoxy.toml Config Parser

[fuunshi]

Goal: Allow Repoxy to read routing configuration from a TOML file.

Tasks:

• Define the format for repoxy.toml:
  • Each route has a path and a list of backends
• Parse the TOML file using serde + toml crate
• Validate that backend URLs are properly formatted
• Load configuration at startup and store it in Rust structs
• Add unit tests for parsing and validation
• Log loaded routes for debugging