[EuiButtonGroup] Better differentiate the current selection

**Is your feature request related to a problem? Please describe.**
[internal feedback] I cannot tell with confidence which value is actually selected by looking at this unless I click and make the selection. But I just want to know what's selected, not change it.

**Describe the solution you'd like**
Can I suggest to make the selected item have more differentiation, perhaps an outline in addition to darker colouring. (this is one example to highlight).

### Proposed redesign

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**Additional context**
This is particularly problematic when only two options are presented (see below):


For reference, we apply margin around the currently active option in the `compressed` version of this input. Perhaps we can apply a similar style to the default size `EuiButtonGroup`, as well.