[EuiComboBox] [EuiSelectable] allow for highlighting a custom fragment when using custom option matcher

[mbondyra]

While testing #7709 I noticed that while passing a custom optionMatcher we cannot highlight the substring that is matched. It'd be good to add it in some way.

Here the fragment get highlighted:

Here it is not, as it comes from the custom matcher check:

[cee-chen]

@tkajtoch do you mind baking this into #7709? Also @mbondyra feel free to leave this as a comment on the PR itself since it hasn't been merged yet!

edit: Oh wait, super sorry, I think I misread the ask. This makes sense to do as a follow-up PR instead of in the same PR