Add Examples Showing Usage with Rosbag Crate

Had a couple users trip over the implementation detail that ros messages over the wire contain the "total message length" as their first 4 bytes, but rosbag and mcap files seem to omit to the total message length.

Have some examples for writing to a bag and reading from a bag with roslibrust.

https://github.com/newpavlov/rosbag-rs

Same with:

https://docs.rs/mcap/latest/mcap/