[roll third_party/pigweed] pw_transfer: Use files in integration tests

This updates pw_transfer's integration tests to use local files as
transfer targets. This makes it simpler to coordinate between C++ and
Python and verify the results of transfer operations.

As part of this, a stream reader and writer that operate on files using
the C++ standard library are added to pw_stream.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/57342

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 416e0f4c97ba356..fad8e85548de5af
Roller-URL: https://ci.chromium.org/b/8837170400578214897
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8837170400578214897
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I48259484af0b07fa59f0f4816f0cb4b628f2ee17
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/59601
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 416e0f4..fad8e85 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 416e0f4c97ba35689266e5ca61148892f35abc99
+Subproject commit fad8e85548de5af196c4e824fd37d0f0518c8247