Add backpressure aware async sequence (#1687)

Motivation:

Swift doesn't have a general purpose backpressure aware async sequence
yet. However, one has been pitched in SE-0406. While we can't use it
yet, we can vendor it in.

Modifications:

- Vendor in the code from SE-0406 with some minor modifications, such as
  changing the name and making it internal

Result:

We have a backpressure aware async sequence.

NOTICES.txt

@@ -28,3 +28,13 @@ It also uses derivations of SwiftNIO's lock 'NIOLock.swift' and locked value box
     * https://github.com/apple/swift-nio/blob/main/LICENSE.txt
   * HOMEPAGE:
     * https://github.com/apple/swift-nio
+
+---
+
+This product contains a derivation of the backpressure aware async stream from
+the Swift project.
+
+  * LICENSE (Apache License 2.0):
+    * https://github.com/apple/swift/blob/main/LICENSE.txt
+  * HOMEPAGE:
+    * https://github.com/apple/swift