pw_protobuf: Skip unknown fields when struct decoding
pw_protobuf: Skip unknown fields when struct decoding

Protobuf parsers are expected to skip unknown fields, and it turns out
in practice encoders sometimes skip extra fields of their own in using
this as a guarantee.

Change StreamDecoder::Read to skip unknown fields, and file a TODO bug
to figure out how to allow them to be inspected and serialized back out
again later.

Change-Id: If1d74174522178bc13e64162dc67c7c2e8b8c713
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/92020
Pigweed-Auto-Submit: Scott James Remnant <6b88cbe64f92c5d64ffbf57d7b3d2efc0467fe34@google.com>
Reviewed-by: Armando Montanez <97ef6fdb231c129ac8827d9f0c4106bfbf57f74f@google.com>
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed