Python API: client.get_raw_messages()

[wkalt]

From the private data platform repo (https://github.com/foxglove/data-platform/issues/676):

In addition to the current client.get_messages() method, it would be useful in some cases such as message statistics gathering to skip decoding and just return tuples of (topic, raw mcap record) from a client.get_raw_messages() method.