Rviz tool to get the cost of a costmap cell

[tonynajjar]

Feature request

Feature description

Sometimes when debugging planners or controllers it's useful to know the cost at a particular point/region in the costmap.
We could have an Rviz tool that allows to click on a particular cell in the costmap and get its cost. We also would need to expose a service in costmap_2d_ros that retrieves this cost.

Implementation considerations

A first attempt was done in this PR but not completed

[SteveMacenski]

Merging imminent in #4546