Allow specifying a custom executor

[mtkennerly]

As discussed on Discord, with the move away from the Application trait, we lost the ability to set a custom Executor. This PR restores that functionality.

Test case:

Diff

diff --git a/examples/clock/Cargo.toml b/examples/clock/Cargo.toml
index bc6c202b..fb8053d4 100644
--- a/examples/clock/Cargo.toml
+++ b/examples/clock/Cargo.toml
@@ -10,3 +10,4 @@ iced.workspace = true
 iced.features = ["canvas", "tokio", "debug"]
 chrono = "0.4"
 tracing-subscriber = "0.3"
+tokio.workspace = true
diff --git a/examples/clock/src/main.rs b/examples/clock/src/main.rs
index ef3064c7..0a97803b 100644
--- a/examples/clock/src/main.rs
+++ b/examples/clock/src/main.rs
@@ -15,6 +15,7 @@ pub fn main() -> iced::Result {
         .subscription(Clock::subscription)
         .theme(Clock::theme)
         .antialiasing(true)
+        .executor::<Executor>()
         .run()
 }

@@ -166,3 +167,28 @@ fn hand_rotation(n: u32, total: u32) -> Degrees {

     Degrees(360.0 * turns)
 }
+
+pub struct Executor(tokio::runtime::Runtime);
+
+impl iced::Executor for Executor {
+    fn new() -> Result<Self, iced::futures::io::Error> {
+        println!("Custom executor is active");
+        tokio::runtime::Builder::new_multi_thread()
+            .enable_all()
+            .build()
+            .map(Self)
+    }
+
+    #[allow(clippy::let_underscore_future)]
+    fn spawn(
+        &self,
+        future: impl std::future::Future<Output = ()> + Send + 'static,
+    ) {
+        let _ = tokio::runtime::Runtime::spawn(&self.0, future);
+    }
+
+    fn enter<R>(&self, f: impl FnOnce() -> R) -> R {
+        let _guard = tokio::runtime::Runtime::enter(&self.0);
+        f()
+    }
+}

Output

$ cargo run -p clock
   Compiling clock v0.1.0 (C:\git\_forks\iced\examples\clock)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 4.28s
     Running `target\debug\clock.exe`
Custom executor is active

[hecrj]

Thanks! Looks great.