[Windows] Add a sandbox string class for low-level heap allocations.

This CL adds a class to represent a low-level heap allocated string.
In addition the class is then used in various NT utility functions
which return strings with corresponding changes to the call sites for
those utilities.

Bug: 447171244
Change-Id: Iaaa714707b4476582b34aa12a4e99522ec1a9a41
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7000096
Reviewed-by: Alex Gough <ajgo@chromium.org>
Commit-Queue: James Forshaw <forshaw@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1524571}