roll: third_party/pigweed pw_rpc2: Packet framing and encoding/decoding

Introduce core packet framing and wire-protocol definitions for pw_rpc2,
including InboundPacket, OutboundPacket, and HandshakePacket.

This defines the fundamental wire-level header structures and handles
encoding and zero-copy decoding/slicing for RPC packets.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/480285
Original-Revision: ab15f40ddfeeae02c50a298c22e3a65e2484dcd5

Rolled-Repo: https://pigweed.googlesource.com/pigweed/pigweed
Rolled-Commits: 4491a8aa376777..ab15f40ddfeeae
Roll-Count: 1
Roller-URL: https://cr-buildbucket.appspot.com/build/8669976018363984961
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I6405e72af3798159862493c0a32daad5092e31e1
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/482505
diff --git a/third_party/pigweed b/third_party/pigweed
index 4491a8a..ab15f40 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 4491a8aa376777957c36281665ae47506ef4c328
+Subproject commit ab15f40ddfeeae02c50a298c22e3a65e2484dcd5