[roll third_party/pigweed] pw_file: FlatFileSystem service implementation

Provides an optional implementation for the FileSystem RPC service that
enumerates a list of files as a "flat" file system.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/60401

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: eaf1e547f52b390..5be54582d9e51e7
Roller-URL: https://ci.chromium.org/b/8835811832962984497
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8835811832962984497
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Idd21e8ed86bbd109af672283bf8ec2624a927c46
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/61381
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 eaf1e54..5be5458 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit eaf1e547f52b39005fad42ee04f36084859109e3
+Subproject commit 5be54582d9e51e79b81ca31e006ff579f27242f1