Add support for configuring intensity of lights

[ahcorde]

Add support for configuring intensity of lights

[ahcorde]

• sdformat Added lights intensity field sdformat#484
• ign-msgs Added light intensity to light.proto gz-msgs#131
• ign-rendering Added Light Intensity #233
• ign-gazebo Added light intensity gz-sim#612

[ahcorde]

Diferrent point light intensities:

• Ogre1

• Ogre2

[iche033]

can you add SetIntensity(1.0) to the Reset function?

[ahcorde]

All PR are merged. Closing.