CLI: mcap compress subcommand

[wkalt]

Currently you can recompress a file using the filter or merge commands, but to a user who wants to recompress these aren't very discoverable. Rosbag provides a "compress" command that does more or less what we would need - although it allows you to compress multiple files in one call, which we could maybe consider dropping (not clear if it's the tool's job or should be done in bash).