SITL/Gazebo: Add support for airspeed and battery voltage simulation.

I've been testing using the Gazebo SITL and things are flying. However I've noticed that the reported battery voltage and reported airspeeds are zero.

I have some downstream tools which use those values. I'd like to add them to the SITL however I am not sure exactly where to start to add these values.

Using other SITL instances I've used the battery start at 100% and drift down to ~ 20% after a few minutes and then just stay there. Which would be adequate for my use case.

And for airspeed I'm guessing that the value is simply getting a default value.

I'll be looking into adding this soon. But if I could get some suggestions of what approaches would be best that would be appreciated. @khancyr @rmackay9 @tridge

Start by locating the Gazebo SITL sensor integration and comparing how other SITL instances provide battery and airspeed values. Done means the Gazebo SITL reports non-zero simulated battery voltage and airspeed, with battery behavior matching the requested drift toward approximately 20%.

Written by the indexing model from the issue text.