feat: Add #[main] and #[dispatch] macros for type-safe guest entry points

[jprendes]

Summary

This PR introduces #[main] and #[dispatch] attribute macros for defining guest entry points, replacing the previous pattern of manually exporting extern "C" functions with #[no_mangle].

Changes

Make entry points optional

• hyperlight_main and guest_dispatch_function are now optional using weak symbols with sensible defaults:
  • hyperlight_main defaults to a no-op
  • guest_dispatch_function defaults to returning a GuestFunctionNotFound error

Add attribute macros

• #[main] - Marks a function as the guest's initialization entry point
• #[dispatch] - Marks a function as the fallback handler for unregistered function calls

Example

Before:

#[no_mangle]
pub extern "C" fn hyperlight_main() {
    // initialization code
}

#[no_mangle]
pub fn guest_dispatch_function(function_call: FunctionCall) -> Result<Vec<u8>> {
    // dispatch logic
}

After:

#[hyperlight_guest_bin::main]
fn main() {
    // initialization code
}

#[hyperlight_guest_bin::dispatch]
fn dispatch(function_call: FunctionCall) -> Result<Vec<u8>> {
    // dispatch logic
}

Benefits

• Ergonomic API: No need for #[no_mangle] or extern "C" boilerplate
• Optional entry points: Guests that don't need initialization or custom dispatch no longer need to define stub functions
• Future-proof: The macro approach enables compile-time function signature validation in the future, catching mismatches early rather than at link time or runtime