[roll] Roll fuchsia Reland "[assembly] Add fxfs_pbtool to the product bundle."

This relands the previous ffx update with a modified strategy to ensure
compatibility. In this commit, ffx is updated so that it can safely read
the new format, but the actual PB definitions are not changed yet.

Rollout Plan: (This commit): Update ffx to support reading the new PB
format. (Future commit): Once this ffx update is fully rolled out
  everywhere, update the actual PBs so they contain the new field.

This is a reland of commit f54f1124914e579a1989c246821e7e55ca833447

Original change's description:
> [assembly] Add fxfs_pbtool to the product bundle.
>
> Add a new function to tool provider that returns a
> tool path. Use that function to access fxfs_pbtool in
> an assembled system. Copy the tool path into the PB.
>
> Original-Bug: b/472511715
>
> Change-Id: I59ee46840d718f5be076b528011900c24adfc67e
> Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1508680
> Reviewed-by: David Iserovich <diserovich@google.com>
> Reviewed-by: Aidan Wolter <awolter@google.com>
> Reviewed-by: Clayton Wilkinson <wilkinsonclay@google.com>
> Commit-Queue: Ida Isugi <isugiidap@google.com>

Original-Bug: b/472511715
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1527916
Original-Revision: c7935460ac5969cda1507c5ad8f03eaf5efa8f8c
GitOrigin-RevId: 707449ece51658b7846a1cbc91fa39a31735ad27
Change-Id: Idbdacc9752008cf1c14d45923c1f33646957312b