[roll pigweed] pw_watch: Make httpwatcher optional

httpwatcher 0.5.2 requires tornado <5, which is not compatible with
Python 3.10. httpwatcher does not appear to be actively maintained, and
has not been updated since 2018.

To ensure pw_watch works with Python 3.10 without breaking existing
users of the --serve-docs option, make the httpwatcher import optional.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/100662

https://pigweed.googlesource.com/pigweed/pigweed
pigweed Rolled-Commits: de64d37ce9ba923..74144e60b1b5664
Roller-URL: https://ci.chromium.org/b/8809819504194023185
Cq-Cl-Tag: roller-builder:pigweed-zephyr-roller
Cq-Cl-Tag: roller-bid:8809819504194023185
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Icca3f7e2c098e3a92fff3ed626647d65a02c3692
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/zephyr-integration/+/100719
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/pigweed b/pigweed
index de64d37..74144e6 160000
--- a/pigweed
+++ b/pigweed
@@ -1 +1 @@
-Subproject commit de64d37ce9ba9239bb0a79644604f083dfd69b40
+Subproject commit 74144e60b1b56648ccfb7136cbd1a1676ec9e80a