runtime: implement AddCleanup

runtime: implement AddCleanup

This change introduces AddCleanup to the runtime package. AddCleanup attaches
a cleanup function to an pointer to an object.

The Stop method on Cleanups will be implemented in a followup CL.

AddCleanup is intended to be an incremental improvement over
SetFinalizer and will result in SetFinalizer being deprecated.

For #67535

Change-Id: I99645152e3fdcee85fcf42a4f312c6917e8aecb1
Reviewed-on: https://go-review.googlesource.com/c/go/+/627695
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>