[roll third_party/pigweed] pw_channel: Add EpollChannel

This implements a pw::channel::Channel which sends and receives data
through a file descriptor, with read and write notifications backed by
Linux's epoll system.

To facilitate this, several additional APIs are added to the epoll
async2 dispatcher, allowing registration of additional file descriptors.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/210813
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https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 3763a9d5b54c766..e07a08914d79ea1
Roller-URL: https://ci.chromium.org/b/8746555022131683777
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Change-Id: I8334b68784007db998b1dec6be83e444f4225368
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/quickstart/bazel/+/212097
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
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diff --git a/third_party/pigweed b/third_party/pigweed
index 3763a9d..e07a089 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 3763a9d5b54c766855bab5fb68827e2a76276112
+Subproject commit e07a08914d79ea1c7cf3d337b2d5e93ba91c37ee