CLI: return actual copy/pasteable protobuf schemas from mcap list schemas on a protobuf mcap

[wkalt]

Currently when you run mcap list schemas on a protobuf mcap it makes a best effort translation of the binary FileDescriptorSet back to text for display. The translation is sufficient to convey the schema's meaning, but it's not guaranteed to be a valid .proto. From what I remember there are some reasons why it is not straightforward to go from FileDescriptorSet back to text .proto -- but unfortunately I don't remember what those reasons are.

If possible, it would be nice to recover exactly equivalent .proto text schemas to what's in the file descriptorset.