Publish time range in various records

[pauley-unsaturated]

How hard would it be to amend the mcap file format to add publish time range in all of the places where we already have log time range (message_start_time / message_end_time)?

I'm looking at the spec, but I feel like there's a wall at the Message Index, would likely require a PublishTime Index. Also: it would seem prudent to add a Version field to your header when you can but that's a different story :)

[pauley-unsaturated]

The reason we want this is that we want to sort by both message time (log time) and publish time, which we are using as "measurement time", aka the time when the raw input corresponding to this message was believed to be received.