[Heatmap] Add axes titles

[ghudgins]

Motivation / Problem

When I am displaying data on a heatmap
I need to label the two dimensions of the heatmap
So I can optionally provide additional context as to what the data is

Describe the solution you'd like
Expose optional axis labeling options in heatmap visualizations for upstream consumption

Describe alternatives you've considered
None - there's no great workaround today

Kibana Cross Issues
elastic/kibana#102036
elastic/kibana#72390

Checklist

Delete any items that are not applicable to this feature request.

• this request is checked against already exist requests
• every related Kibana issue is listed under Kibana Cross Issues list
• kibana cross issue tag is associated to the issue if any kibana cross issue is present

[markov00]

One aspect that we don't consider much as part of the visualization is the title of the visualization itself.
Most of the time is enough to correctly describe the dimensions in play:

The monthly infection rate per Region/Age

Tshirt orders by color and size

In Kibana the title is actually only confided at the dashboard level, we can bring it back at the editing level.
I don't think the title should change depending on dashboard, it depends on the data and the configuration, so it should stay the same everytime