msgpack support in mcap cat --json output

[jon-chuang]

Similar to #763, msgpack is also a popular schemaless encoding

[defunctzombie]

@jon-chuang Please me mindful when using the bug label. The bug label is for broken functionality. If you believe this issue is a bug please explain what existing functionality that we've documented is not working.

[jtbandes]

@defunctzombie I think it's understandable as there's no way for community members to file a non-bug issue from this page 😄

[jtbandes]

I'm going to convert this to a discussion since we haven't yet settled on the spec updates to "well-known" encodings for msgpack usage.