Is your feature request related to a problem? Please describe.
EuiRange sliders with ticks do not announce the human-readable tick labels to screen readers.

Describe the solution you'd like
Adding a named prop aria-valuetext that reasons about the currently selected radio button and sets `aria-valuetext=CURRENT_TICK.label}".

Additional context
@cleydyr created a CodeSandbox that illustrates how we could derive the prop value. Bringing it into the component as a named prop instead of allowing it via spread props helps make it more consistent.