feat: reintroduce backend.inspect

[nbaztec]

What 💻

• Reintroduces Backed::inspect that was previously removed due to re-entrancy issues.
• An additional inspect_ctx: Box<dyn Any> is now added to Backend::inspect to tightly couple any zk-specific data we'd need to pass.
  • It's not set in ctx as it must not be persisted beyond the call (and also immediately used), which isn't guaranteed due to the additional indirection via Backend::inspect.
  • Additionally, we cannot modify a CowBackend to set inspect_ctx on the inner backend context as it will always clone the &Backend immediately.
  • The issues above are solvable by inlining inspect calls directly in executor (as was done previously) but then we diverge from foundry code.

Why ✋

• Stay close to foundry code.

Evidence 📷

Tests pass