feat(hydro_lang): use mimalloc when compiling for an external host

[shadaj]

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• -> feat(hydro_lang): use mimalloc when compiling for an external host #2268
• fix(hydro_deploy_integration): compilation on non-Unix [ci-full] #2267

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[shadaj]

hey bot, what is this PR about?

[shadaj]

This PR introduces mimalloc as the global memory allocator for Hydro programs compiled for external hosts. Here's what it does:

📌 Key Changes:

• Adds mimalloc dependency to hydro_lang
• Creates a new runtime_mimalloc feature flag
• Configures mimalloc as the #[global_allocator] in generated trybuild code
• Automatically enables mimalloc when the target is a non-local host (i.e., external deployments)
• Includes platform-specific conditional compilation for Unix vs non-Unix systems

⚡ Purpose:
Mimalloc is a high-performance memory allocator that can provide better performance and reduced memory fragmentation compared to the default Rust allocator. By using it for external host deployments, this should improve the runtime performance of Hydro applications running on remote systems.

🔧 Technical Details:

• The feature is conditionally enabled in deploy_graph.rs based on HostTargetType::Local
• Also includes some infrastructure fixes (like adding Arc<TempDir> for better resource management in SingleConnectionSource and SingleConnectionSink)