[Test] Expose the MessageInfo to the GenericSubscription to potentially use to match messages to QoS

[emersonknapp]

Experiment for #629 to see how hard it is to receive message metadata to potentially correlate incoming messages with a specific publisher. Looks very promising

Briefly tested, 2 publishers on /foo, and one on /bar - the print statement in the message callback gives output like:

$ ros2 bag record -a -o bagSeesMessageInfo
[INFO] [1612406764.918839725] [rosbag2_storage]: Opened database 'bagSeesMessageInfo/bagSeesMessageInfo_0.db3' for READ_WRITE.
[INFO] [1612406764.926742967] [rosbag2_transport]: Listening for topics...
[INFO] [1612406764.928994114] [rosbag2_transport]: Subscribed to topic '/rosout'
[INFO] [1612406764.930462463] [rosbag2_transport]: Subscribed to topic '/foo'
[INFO] [1612406764.932862746] [rosbag2_transport]: Subscribed to topic '/parameter_events'
[INFO] [1612406764.934140601] [rosbag2_transport]: Subscribed to topic '/bar'
[INFO] [1612406765.247710249] [rosbag2_transport]: MESSAGE INFO on /bar: pubgid !$g
[INFO] [1612406765.322675283] [rosbag2_transport]: MESSAGE INFO on /foo: pubgid NrE[0m
[INFO] [1612406765.504592903] [rosbag2_transport]: MESSAGE INFO on /foo: pubgid ^b
[INFO] [1612406766.247643351] [rosbag2_transport]: MESSAGE INFO on /bar: pubgid !$g
[INFO] [1612406766.322676287] [rosbag2_transport]: MESSAGE INFO on /foo: pubgid NrE[0m
[INFO] [1612406766.504602200] [rosbag2_transport]: MESSAGE INFO on /foo: pubgid ^b
[INFO] [1612406767.247675681] [rosbag2_transport]: MESSAGE INFO on /bar: pubgid !$g
[INFO] [1612406767.322470858] [rosbag2_transport]: MESSAGE INFO on /foo: pubgid NrE[0m