Eui: EuiFieldSearch's clear button covers the `value` of the input

[afgomez]

When a EuiFieldSearch with clearable={true} contains a long value, the clear button covers covers the content underneath it.

Tested in the current playground https://elastic.github.io/eui/#/forms/form-controls

[cchaos]

Looks like the right side padding was never added to this component with an -isClearable class.

[sg23600]

I wish to work on this issue.