[a11y] Small Multiple title information for screen readers

[rshen91]

Is your feature request related to a problem? Please describe. Opening this issue to build off of https://github.com/elastic/elastic-charts/pull/1472 to address small multiples in cases where the vertical axes are split. In these instances, the titles are more complicated than the general cartesian charts. Instead of the titles being available on the axis spec, the information is in a separate GroupBy component.

This issue is open to build off of #1472 to provide more semantic context for screen reader users in cases with small multiples.

• [x] 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