Update combobox option's value TS definition

[chandlerprall]

Summary

Fixes #2078 by updating the typescript definition for EuiComboBox's options' value.

I tested these changes in a new project to verify the generic was understood properly -

I also dropped the resulting eui.d.ts file into Kibana and ran the typecheck script, there are two instances in x-pack/legacy/plugins/siem/public/components/edit_data_provider/index.tsx where EuiComboBox is used through styled-components which interprets the generic as {} which is invalid. Both cases are solved by moving the css style onto EuiComboBox as a prop.

Checklist

- [ ] This was checked in mobile
- [ ] This was checked in IE11
- [ ] This was checked in dark mode
- [ ] Any props added have proper autodocs
- [ ] Documentation examples were added

• A changelog entry exists and is marked appropriately
• This was checked for breaking changes and labeled appropriately
  - [ ] Jest tests were updated or added to match the most common scenarios
  - [ ] This was checked against keyboard-only and screenreader scenarios
  - [ ] This required updates to Framer X components

[thompsongl]

LGTM. Confirmed the same result/required action in Kibana.

[chandlerprall]

@thompsongl I switched the generic to have a default value, un-doing the breaking aspect of this change.

[thompsongl]

Looks like just 2 updates required in Kibana after these changes, both of which are related to styled-components and not real TS errors.