Standard name for raw pb message/data in wrapper classes

[llucax]

Coming from frequenz-floss/frequenz-client-reporting-python#16 (comment):

The PR calls it _data_pb, the SDK _raw. It would be nice to agree on some standard so all clients use the same, although it is not exposed so it is not that important.

Maybe _pb_message is the most accurate approach, or just _pb if we want to be extra terse, as it is only a private attribute.

[Marenz]

I vote for _pb