pw_protobuf: return a mutable BytesReader
pw_protobuf: return a mutable BytesReader

Change-Id: I9f935d385babcccb62f679f7cd575ad79bf19434
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/62083
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Yecheng Zhao <858a3f9ab0d064c70af0dcd6105b38f356960c6a@google.com>
Pigweed-Auto-Submit: Ewout van Bekkum <638f134dbc68778d6b859fb04a64baea2b00b021@google.com>

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