Adopt Sendable in ChannelHandlers.swift (apple#2144)

Co-authored-by: Cory Benfield <[email protected]>

Sources/NIOCore/ChannelHandlers.swift

@@ -105,6 +105,11 @@ public final class AcceptBackoffHandler: ChannelDuplexHandler, RemovableChannelH
     }
 }
 
+#if swift(>=5.7)
+@available(*, unavailable)
+extension AcceptBackoffHandler: Sendable {}
+#endif
+
 /**
  ChannelHandler implementation which enforces back-pressure by stopping to read from the remote peer when it cannot write back fast enough.
  It will start reading again once pending data was written.
@@ -152,6 +157,11 @@ public final class BackPressureHandler: ChannelDuplexHandler, RemovableChannelHa
     }
 }
 
+#if swift(>=5.7)
+@available(*, unavailable)
+extension BackPressureHandler: Sendable {}
+#endif
+
 /// Triggers an IdleStateEvent when a Channel has not performed read, write, or both operation for a while.
 public final class IdleStateHandler: ChannelDuplexHandler, RemovableChannelHandler {
     public typealias InboundIn = NIOAny
@@ -160,7 +170,7 @@ public final class IdleStateHandler: ChannelDuplexHandler, RemovableChannelHandl
     public typealias OutboundOut = NIOAny
 
     ///A user event triggered by IdleStateHandler when a Channel is idle.
-    public enum IdleStateEvent {
+    public enum IdleStateEvent: NIOSendable {
         /// Will be triggered when no write was performed for the specified amount of time
         case write
         /// Will be triggered when no read was performed for the specified amount of time
@@ -336,3 +346,8 @@ public final class IdleStateHandler: ChannelDuplexHandler, RemovableChannelHandl
         scheduledAllTask = nil
     }
 }
+
+#if swift(>=5.7)
+@available(*, unavailable)
+extension IdleStateHandler: Sendable {}
+#endif