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 <[email protected]>
Reviewed-by: Armando Montanez <[email protected]>
Commit-Queue: Auto-Submit <[email protected]>

pw_protobuf/stream_decoder.cc

@@ -515,13 +515,15 @@ Status StreamDecoder::Read(std::span<std::byte> message,
   PW_TRY(status_);
 
   while (Next().ok()) {
-    // If the field is not found in the table, immediately return NotFound()
-    // without advancing the cursor to allow the caller to inspect the field.
+    // Find the field in the table,
     // TODO(pwbug/650): Finding the field can be made more efficient.
     const auto field =
         std::find(table.begin(), table.end(), current_field_.field_number());
     if (field == table.end()) {
-      return Status::NotFound();
+      // If the field is not found, skip to the next one.
+      // TODO(pwbug/659): Provide a way to allow the caller to inspect unknown
+      // fields, and serialize them back out later.
+      continue;
     }
 
     // Calculate the span of bytes corresponding to the structure field to