context: add AfterFunc

context: add AfterFunc

Add an AfterFunc function, which registers a function to run after
a context has been canceled.

Add support for contexts that implement an AfterFunc method, which
can be used to avoid the need to start a new goroutine watching
the Done channel when propagating cancellation signals.

Fixes #57928

Change-Id: If0b2cdcc4332961276a1ff57311338e74916259c
Reviewed-on: https://go-review.googlesource.com/c/go/+/482695
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Sameer Ajmani <sameer@golang.org>