[cli] Add the ability to dump raw message payloads

[jhurliman]

mcap cat only prints up to the first 10 bytes of raw message payloads (as integers). I want to extract one or more message payloads from an MCAP file and dump them into a binary file. I could see this being an optional flag to the cat command that outputs raw bytes instead of the message summary lines.

[jtbandes]

How would you identify a single message, or what would you imagine happening if more than one message were selected? (like I imagine it could be quite confusing to interpret the resulting file if it contained multiple messages concatenated together)

[jhurliman]

All of the matching messages concatenated into a single binary stream would be fine with me, I can use start/end timestamps to get exactly one message or however many I want. The tool I’ve written myself to fill this need writes each message to a .bin file, but it’s more application specific (it assumes the message payloads are binary and that there will be fewer than 10k of them!).