cpp: read messages in deterministic order, yield message offset in MessageView

[james-rms]

This PR contains a few changes required to get the rosbag2_storage_mcap plugin to actually behave correctly with respect to message ordering and selecting messages by time range.

• Messages with the same timestamp are returned in the order of their position in the file. This is neccessary to have a well-defined return order from ReadMessages, since log timestamps are not required to be unique.
• MessageView includes a new "messageOffset" field, informing the reader of the returned messages' offset within the file. This allows readers to tell if a message returned from one iterator is the same message as returned from a different iterator.

Testing

• unit test added to test new read message order.

[jhurliman]

LGTM. It would be helpful to add a test where there are two chunks, A and B, but the time range of B is partially or fully before A.