[Lens] add better accessibility descriptions to elastic-charts

[ppisljar]

Currently Elastic charts provide very basic description of the content (for example just "bar chart") which is not very useful for screenreaders.

This should be improved so that each chart describes all the used dimensions and list name of dimension, field, operation and values.
For each chart type a proper template should be defined that will allow efficiently describing the content.

Example

Gauge chart measuring {metric description}, with a minimum value of {min description}, a maximum value of {max description} and a goal of {goal description}.

Charts

• gauge chart
• xy chart
• metric chart
• table
• heatmap
• pie

[elasticmachine]

Pinging @elastic/kibana-visualizations (Team:Visualizations)

[elasticmachine]

Pinging @elastic/kibana-accessibility (Project:Accessibility)

[markov00]

@walterra could you update this with the preliminary step you have done till now