Add cross-platform build scripts and infrastructure

[thanos]

This commit adds comprehensive cross-platform build and testing infrastructure for rust-bottle, enabling automated builds across all supported platforms from a single macOS/ARM development machine.

New Features:

• Cross-platform build scripts for macOS/ARM, macOS/x86_64, Linux/x86_64, and Linux/ARM64
• Docker and Podman support for Linux builds with automatic runtime detection
• Dockerfiles for Linux x86_64 and ARM64 builds using latest Rust
• Prerequisites checking script to verify build environment setup
• Quick build test script for fast compilation verification
• Alternative build script using cross tool for efficient cross-compilation
• Comprehensive build documentation in scripts/README_BUILD.md

Infrastructure:

• Added .cargo/config.toml for cross-compilation linker configuration
• Updated .gitignore to allow .cargo/config.toml while ignoring other cargo files
• Docker images use rust:latest to support Cargo.lock v4 format
• All scripts are bash 3.2+ compatible for macOS compatibility

Build Scripts:

• build-all-platforms.sh: Orchestrates builds for all platforms
• build-macos-arm.sh: Native macOS/ARM builds
• build-macos-x86_64.sh: macOS/x86_64 builds using Rosetta 2
• build-linux-x86_64.sh: Linux/x86_64 builds using Docker/Podman
• build-linux-arm64.sh: Linux/ARM64 builds using Docker/Podman
• build-using-cross.sh: Alternative using cross tool
• check-prerequisites.sh: Verify required tools and setup
• quick-build-test.sh: Quick compilation test without full test suite

Docker Support:

• Dockerfile.linux-x86_64: Linux x86_64 build environment
• Dockerfile.linux-arm64: Linux ARM64 build environment with cross-compilation
• .dockerignore: Excludes unnecessary files from Docker builds
• Automatic detection of Docker vs Podman (prefers Podman if available)

Documentation:

• scripts/README_BUILD.md: Complete guide for cross-platform builds
• Includes troubleshooting, prerequisites, and usage examples
• Documents Docker vs Podman differences and usage

This infrastructure enables:

• Automated testing across all target platforms
• CI/CD integration for multi-platform builds
• Easy verification of cross-compilation setup
• Consistent build environments across development machines