[gazebo_plugins] Publish camera info properly for depth cameras.

[ksatyaki]

Aims to address #569

[ksatyaki]

Has this been addressed already? Any info?

[j-rivero]

Has this been addressed already? Any info?

I don't think so. Some comments after giving a quick look into the PR:

• You are removing some protected functions so we are altering the API in the PR. Since kinetic is a mature release I'm not in favour of ABI changes to it. If you are interested into getting the changes in kinetic we should come up with changes that respect the ABI. API changes can go into the next Melodic release.
• I would like to include a test or at least an example that somehow can be run to demonstrate that we are not breaking camera support and that helps to verify that the new information is indeed being published.

Thanks!