Address issues related to `DataTransmitMessage`'s `data` field type and handling of binary data

[aaraney]

The original title for this issue was Change DataTransmitMessage's data field type to accommodate binary data. See discussion here and here.

It relates to #424 and may still have implications on #432.

However, as noted below, we cannot simply change the data field from str to bytes, as this breaks the implementation for serialization. That said, this leaves in question whether/how we can support anything beyond unicode data. This issue should track further discussion and work on addressing this.

[robertbartel]

As noted over in the other issue comments, it looks like this would break JSON serialization for DataTransmitMessage.

[aaraney]

Yeah you are right, @robertbartel. I was not using all of my brain when I was writing this up 😅. There does seem to be a larger issue that we cannot upload binary data without first encoding it in something like base64 or base85, but we should address that in a separate issue.