Auto merge of #96875 - SabrinaJewson:noop-waker, r=m-ou-se

Add `task::Waker::noop`

I have found myself reimplementing this function many times when I need a `Context` but don't have a runtime or `futures` to hand.

Prior art: [`futures::task::noop_waker`](https://docs.rs/futures/0.3/futures/task/fn.noop_waker.html) and [`futures::task::noop_waker_ref`](https://docs.rs/futures/0.3/futures/task/fn.noop_waker_ref.html)

Tracking issue: rust-lang/rust#98286

Unresolved questions:
1. Should we also add `RawWaker::noop()`? (I don't think so, I can't think of a use case for it)
2. Should we also add `Context::noop()`? Depending on the future direction `Context` goes a "noop context" might not even make sense in future.
3. Should it be an associated constant instead? That would allow for `let cx = &mut Context::from_waker(&Waker::NOOP);` to work on one line which is pretty nice. I don't really know what the guideline is here.

r? rust-lang/libs-api `@rustbot` label +T-libs-api -T-libs

Author: bors