[roll third_party/pigweed] pw_transfer: Use RPC writers

This removes the custom virtual writer class from pw_transfer's contexts
in favor of the rpc::Writer base.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/70601

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 4b5ad7d97e5eefc..5e3cf38a468819d
Roller-URL: https://ci.chromium.org/b/8829661235081873169
Cq-Cl-Tag: roller-builder:pigweed-sample-project-roller
Cq-Cl-Tag: roller-bid:8829661235081873169
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ie380dd779cd88118e1e9a3c1b07af8ab1828ae55
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/70720
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 4b5ad7d..5e3cf38 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 4b5ad7d97e5eefc928081268c3e0a059b4aa369b
+Subproject commit 5e3cf38a468819dc662b31d2d19c5560b1081468