Add light map field to material.proto

[iche033]

needed by ign-gazebo so we can convert the lightmap uri in SDF DOM to protobuf msgs and vice versa.

This would be an ABI breaking change.

Related pull request: gazebosim/gz-sim#471

[chapulina]

If #108 gets in and we end up deciding to use ign-msgs7 on Edifice, we could get this in too.

[chapulina]

Edifice will use ign-msgs7. This was addressed on #111.