[Lens] (Accessibility) Field search gives no indication to screen readers that anything is happening

[myasonik]

The problem

When searching for a field name, there are visual changes to the page (the fields filter down based on what you're searching) but there is no indication to a screen reader than anything is changing.

The fix

After the search field, add what's called an "aria-live region" to describe what's happening. This update isn't critical, so we should be polite about it. And it's supplementary to the input so we should also wire that up.

Pseudo-code

<input data-test-subj="lnsIndexPatternFieldSearch" aria-describedby={uniqueId} />
<div role="region" aria-live="polite" id={uniqueId}>
  <FormattedMessage 
    defaultMessage="{availableFields} available fields. {emptyFields} empty fields. {metaFields} meta fields."
    values={{ availableFields, emptyFields, metaFields }}
  />
</div>

WCAG Criteria: 3.2.2 On Input - WCAG A

[elasticmachine]

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

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)