pw_protobuf: allow StreamDecoder subclasses to move
pw_protobuf: allow StreamDecoder subclasses to move

Code generated subclasses will be able to be initialized with an
existing StreamDecoder, moving its members to themselves, in the same
way that StreamEncoder works.

Change-Id: I5874615937895cb7d410432da04c8b2b4ad24209
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/86400
Reviewed-by: Alexei Frolov <a1b001416a8cf9aaea32a3411f8f3c0793ebc0b5@google.com>
Commit-Queue: Scott James Remnant <6b88cbe64f92c5d64ffbf57d7b3d2efc0467fe34@google.com>

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