[roll third_party/pigweed] pw_log_rpc: Add unit tests, docs, & error handling

Add pw_log_rpc unit tests and documentation.
Add an enum to dictacte how to handle server writer errors in
RpcLogDrain.

Test: unit tests pass
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/56931

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: ff2ca8380ef3cf2..c0cbc16ec823caa
Roller-URL: https://ci.chromium.org/b/8836612148133188081
Cq-Cl-Tag: roller-builder:pigweed-sample-project-roller
Cq-Cl-Tag: roller-bid:8836612148133188081
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I0789ebb61515abc46eb5a69794477e26bc7a366b
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/60248
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 ff2ca83..c0cbc16 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit ff2ca8380ef3cf20ec78c1b217d88f77ed236f89
+Subproject commit c0cbc16ec823caa773c4d1a463dffea16240612f