Use `track_caller` for `SpawnExt` (and maybe `LocalSpawnExt`)

[opara-tor]

The Spawn trait provides an abstract interface for spawning tasks. We use this in Arti to allow users to swap the underlying runtime between tokio, async-std, and smol. One problem that we've run into is that the SpawnExt helper interface does not work with tokio-console when using tokio. To provide the code source location of where a task was spawned, tokio relies on the #[track_caller] attribute being applied to the spawn methods. Since SpawnExt::spawn() doesn't use #[track_caller], tokio-console always reports the SpawnExt::spawn() as the source of the task. For example it will report something like:

⚠ This task has woken itself for more than 50% of its total wakeups (51%)
Location: <cargo>/futures-util-0.3.31/src/task/spawn.rs:53:14

If #[track_caller] is added to the SpawnExt::spawn() definition, then tokio-console works as expected:

⚠ This task has woken itself for more than 50% of its total wakeups (51%)
Location: /tmp/arti/crates/tor-circmgr/src/lib.rs:522:14

We can't add #[track_caller] ourselves because we can't provide our own impl of SpawnExt (trait coherence).

I think it would be useful to add #[track_caller] to SpawnExt::spawn() and SpawnExt::spawn_with_handle(). And possibly for LocalSpawnExt as well.

Would you be interested in this change? If so are there any futures-util 0.3.x releases on the 0.3 branch planned that we could add this to? Or should this go on main only?