Add wing force sensors to fruitfly_v2 walker

[vaxenburg]

This PR adds force sensors to fruitfly_v2 walker's wings to approximate wing load sensors of the real fruit fly.

[yuvaltassa]

Do you want force sensors or torque sensors? The latter makes more sense to me.

Also, note this recommendation from the documentation

Using this sensor often requires creating a dummy body welded to its parent (i.e., having no joint elements).

Usually when using force or torque sensors on jointed bodies you get unexpected values. I recommend introducing a dummy static body at the joint root (child of the thorax), attaching the sensor to this body, and making the wing a child of this body.