EuiCard should use role=heading where appropriate

[timroes]

Currently EuiCard never uses a proper header. In case the whole card is a button that is fine (and everything else would be breaking with HTML spec).

In case we actually render the button inside the card, I think it would make sense to make the (visual) header an actual HTML header, most likely by using role="heading" on it and allow the user to customize which level it sits (which should be set via aria-level in that case).

cc @bhavyarm @nreese