feat(swarm): Make executor for connection tasks explicit (#3097)

Previously, the executor for connection tasks silently defaulted to a `futures::executor::ThreadPool`. This causes issues such as https://github.com/libp2p/rust-libp2p/issues/2230.

With this patch, we force the user to choose, which executor they want to run the connection tasks on which results in overall simpler API with less footguns.

Closes #3068.

swarm/src/executor.rs

@@ -0,0 +1,48 @@
+use futures::executor::ThreadPool;
+use std::{future::Future, pin::Pin};
+
+/// Implemented on objects that can run a `Future` in the background.
+///
+/// > **Note**: While it may be tempting to implement this trait on types such as
+/// >           [`futures::stream::FuturesUnordered`], please note that passing an `Executor` is
+/// >           optional, and that `FuturesUnordered` (or a similar struct) will automatically
+/// >           be used as fallback by libp2p. The `Executor` trait should therefore only be
+/// >           about running `Future`s on a separate task.
+pub trait Executor {
+    /// Run the given future in the background until it ends.
+    fn exec(&self, future: Pin<Box<dyn Future<Output = ()> + Send>>);
+}
+
+impl<F: Fn(Pin<Box<dyn Future<Output = ()> + Send>>)> Executor for F {
+    fn exec(&self, f: Pin<Box<dyn Future<Output = ()> + Send>>) {
+        self(f)
+    }
+}
+
+impl Executor for ThreadPool {
+    fn exec(&self, future: Pin<Box<dyn Future<Output = ()> + Send>>) {
+        self.spawn_ok(future)
+    }
+}
+
+#[cfg(feature = "tokio")]
+#[derive(Default, Debug, Clone, Copy)]
+pub(crate) struct TokioExecutor;
+
+#[cfg(feature = "tokio")]
+impl Executor for TokioExecutor {
+    fn exec(&self, future: Pin<Box<dyn Future<Output = ()> + Send>>) {
+        let _ = tokio::spawn(future);
+    }
+}
+
+#[cfg(feature = "async-std")]
+#[derive(Default, Debug, Clone, Copy)]
+pub(crate) struct AsyncStdExecutor;
+
+#[cfg(feature = "async-std")]
+impl Executor for AsyncStdExecutor {
+    fn exec(&self, future: Pin<Box<dyn Future<Output = ()> + Send>>) {
+        let _ = async_std::task::spawn(future);
+    }
+}