Refactor: Preserve Raw Message in Message Parser

[CanardConfit]

Updated the parse method to ensure that raw_message refers to the original unprocessed message received by the parser.

This avoids unintended transformations, preventing the need for explicit casting when accessing raw_message. Renamed raw_message parameter to raw_message_str to clarify its nature before JSON deserialization.

[Era-Dorta]

Thanks for the PR.