[cli] Add `mcap cat --json` support for unsupported/unrecognized encodings

[jhurliman]

Today:

$ mcap cat --topics /not/a/proto --json data.mcap | head -n1
2023/02/16 14:37:12 Error: failed to print messages: JSON output only supported for ros1msg, protobuf, and JSON schemas

The future:

$ mcap cat --topics /not/a/proto --json data.mcap | head -n1
{"topic":"/not/a/proto","sequence":0,"log_time":1672783876.875759239,"publish_time":1672783876.875759239,"data":"aGVsbG8sIHdvcmxkCg=="}