pw_router: Extract packet priority and forward metadata to egresses

This adds a priority extractor to the router's PacketParser, and defines
a metadata struct which is forwarded to egresses with optional data
extracted from a packet.

Change-Id: I69f856b900e73de22d1ecb86e912aa2f7a96c436
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/48680
Reviewed-by: Wyatt Hepler <hepler@google.com>