swarm: Split off "keep alive" functionality from DummyConnectionHandler (#2859)

Previously, the `DummyConnectionHandler` offered a "keep alive" functionality,
i.e. it allowed users to set the value of what is returned from
`ConnectionHandler::keep_alive`. This handler is primarily used in tests or
`NetworkBehaviour`s that don't open any connections (like mDNS). In all of these
cases, it is statically known whether we want to keep connections alive. As
such, this functionality is better represented by a static
`KeepAliveConnectionHandler` that always returns `KeepAlive::Yes` and a
`DummyConnectionHandler` that always returns `KeepAlive::No`.

To follow the naming conventions described in
https://github.com/libp2p/rust-libp2p/issues/2217, we introduce a top-level
`keep_alive` and `dummy` behaviour in `libp2p-swarm` that contains both the
`NetworkBehaviour` and `ConnectionHandler` implementation for either case.

protocols/ping/src/handler.rs

@@ -111,6 +111,10 @@ impl Config {
     /// If the maximum number of allowed ping failures is reached, the
     /// connection is always terminated as a result of [`ConnectionHandler::poll`]
     /// returning an error, regardless of the keep-alive setting.
+    #[deprecated(
+        since = "0.40.0",
+        note = "Use `libp2p::swarm::behaviour::KeepAlive` if you need to keep connections alive unconditionally."
+    )]
     pub fn with_keep_alive(mut self, b: bool) -> Self {
         self.keep_alive = b;
         self