Implement a back-pressure aware AsyncSequence source (#2230)

* Implement a back-pressure aware `AsyncSequence` source

# Motivation
We ran into multiple use-cases (#2067, https://github.com/grpc/grpc-swift/blob/main/Sources/GRPC/AsyncAwaitSupport/PassthroughMessageSource.swift) already where we want to vend an `AsyncSequence` where elements are produced from the sync world while the consumer is in the async world. Furthermore, we need the `AsyncSequence` to properly support back-pressure.
Since we already identified that this is something fundamental for our ecosystem and that current `AsyncSequence` sources are not providing the proper semantics or performance, it would be great to find a single solution that we can use everywhere.

Before diving into the code, I think it is good to understand the goals of this `AsyncSequence`:
- The `AsyncSequence` should support a single unicast `Subscriber`
- The `AsyncSequence` should allow a pluggable back-pressure strategy
- The `AsyncSequence` should allow to yield a sequence of elements to avoid aquiring the lock for every element.
- We should make sure to do as few thread hops as possible to signal the producer to demand more elements.

# Modification
This PR introduces a new `AsyncSequence` called `NIOBackPressuredAsyncSequence`. The goal of that sequence to enable sync to async element streaming with back-pressure support.

# Result
We can now power our sync to async use-cases with this new `AsyncSequence`.

# Future work
There are couple of things left that I wanna land in a follow up PR:
1. An adaptive back-pressure strategy that grows and shrinks depending on the speed of consumption
2. A throwing version of this sequence
3. Potentially an async version that suspends on `yield()` and resumes when more elements should be demanded.

* Fix cancellation handling

* Review

* Add init helper

* Add return types to help the type checker

* Fix 5.7 CI

* Rename delegate method and update docs

* Review

* Switch to Deque, rename the type and change the behaviour of yielding an empty sequence

* Review comments from George

* Review comments by Konrad

* Review and add tests for the high low strategy

* Fix 5.4 tests

* Code review

Co-authored-by: Cory Benfield <[email protected]>

Author: FranzBusch

Package.swift

@@ -16,10 +16,11 @@
 import PackageDescription
 
 let swiftAtomics: PackageDescription.Target.Dependency = .product(name: "Atomics", package: "swift-atomics")
+let swiftCollections: PackageDescription.Target.Dependency = .product(name: "DequeModule", package: "swift-collections")
 
 var targets: [PackageDescription.Target] = [
     .target(name: "NIOCore",
-            dependencies: ["NIOConcurrencyHelpers", "CNIOLinux", "CNIOWindows"]),
+            dependencies: ["NIOConcurrencyHelpers", "CNIOLinux", "CNIOWindows", swiftCollections]),
     .target(name: "_NIODataStructures"),
     .target(name: "NIOEmbedded",
             dependencies: ["NIOCore",
@@ -140,6 +141,7 @@ let package = Package(
     ],
     dependencies: [
         .package(url: "https://github.com/apple/swift-atomics.git", from: "1.0.2"),
+        .package(url: "https://github.com/apple/swift-collections.git", from: "1.0.2"),
     ],
     targets: targets
 )