feat: Promote rpxy-lib/sticky-cookie to top-level feature

.. and fix compile warnings when disabled

rpxy-lib/src/backend/load_balance/load_balance_main.rs

@@ -131,6 +131,4 @@ impl LoadBalance {
 pub struct LoadBalanceContext {
   #[cfg(feature = "sticky-cookie")]
   pub sticky_cookie: StickyCookie,
-  #[cfg(not(feature = "sticky-cookie"))]
-  pub sticky_cookie: (),
 }

rpxy-lib/src/backend/load_balance/mod.rs

@@ -4,6 +4,7 @@ mod load_balance_sticky;
 #[cfg(feature = "sticky-cookie")]
 mod sticky_cookie;
 
+#[cfg(feature = "sticky-cookie")]
 use super::upstream::Upstream;
 use thiserror::Error;
 
@@ -16,6 +17,7 @@ pub use load_balance_sticky::LoadBalanceStickyBuilder;
 pub use sticky_cookie::{StickyCookie, StickyCookieValue};
 
 /// Result type for load balancing
+#[cfg(feature = "sticky-cookie")]
 type LoadBalanceResult<T> = std::result::Result<T, LoadBalanceError>;
 /// Describes things that can go wrong in the Load Balance
 #[derive(Debug, Error)]