Fix: Propagate user attributes through #[jolt::provable] macro

[shreyas-londhe]

Fixes #588

Problem

The #[jolt::provable] proc macro was dropping user-defined attributes (like #[allow(arithmetic_overflow)]) when generating functions, causing compilation errors or unexpected behavior.

Changes

• make_execute_function: Forward self.func.attrs to the generated host function
• make_main_func: Wrap guest code in an inner function that carries the user's attributes (avoids unstable "attributes on expressions" feature)

Testing

Added examples/attribute/ with tests for:

• #[allow(arithmetic_overflow)]
• #[allow(unused_variables)]
• #[inline]
• Multiple attributes combined

[0xAndoroid]

Thank you for your contribution. I believe that general idea is sound, just some parts need to be addressed.
In addition, can we remove the new attribute example all together, just leaving changes to the macro sdk to not bloat the codebase.
Finally, make sure CI passes.

[0xAndoroid]

Because we don't make this a closure anymore, it can't capture input variables from the outside. We still need to keep this a closure.
I believe in rust you can still apply most of the attributes to closures. But it'd be nice to document this limitation in the jolt book.

[shreyas-londhe]

That makes sense, thanks for pointing this out! Unfortunately applying attributes to expressions (closures) is experimental on stable Rust (rust-lang/rust#15701).

We would need to use nightly version to add #![feature(stmt_expr_attributes)] to the generated guest code. Or we can apply the attributes to main which would then propagate down.