Proposal - support velocity and velocity covariance in NavSatFix

[Ryanf55]

Feature request

Feature description

In NavSatStatus, there is a field for position covariance. Some GPS receivers also support velocity with a velocity covariance.
I'd like to request some fields added to report both the velocity and accuracy estimates of that velocity

Implementation considerations

There is some thought in other threads about re-adding the GPSFix message support. I'd like to get the ball rolling on this for ROS2 Iron, if possible, since there are two other tickets regarding the NavSatFix message. Seems like a good opportunity to bundle all the changes in one version bump.