[Synthetics] add aria-live region to add monitor creation flow (elast…

…ic#191135)

## Summary

Resolves
elastic/observability-accessibility#14

### Testing

1. Turn on your screen reader. For Mac, to turn on voiceover it's
Command + F5.
2. Make sure you have a few synthetics monitors created, at least two
3. Navigate to `synthetics/add-monitor`. Tab to monitor type radio list
and use the right and left arrow buttons to change the option.
4. The content of the bottom descriptive callout should be read when
changing the monitor type

x-pack/plugins/observability_solution/synthetics/public/apps/synthetics/components/monitor_add_edit/fields/monitor_type_radio_group.tsx

@@ -92,7 +92,7 @@ export const MonitorTypeRadioGroup = ({
       </EuiKeyPadMenu>
       <EuiSpacer />
       {selectedOption && (
-        <EuiPanel color="primary">
+        <EuiPanel color="primary" aria-live="polite">
           <EuiText size="s">
             <h4>{selectedOption.descriptionTitle}</h4>
           </EuiText>