EuiListGroup gutter prop

**Request**

`EuiListGroup` is not allowing for consumers to [define space between elements](https://github.com/elastic/kibana/pull/286277/changes#diff-5f19c27efe0636ff423e6e2094ea17b4f5328e1862e5ae4ad20b48e337c73229R220), so that way the list would always looks stacked. Let's add a gutter property to the list which would create gap between each element.

Related to this usecase https://github.com/elastic/kibana/pull/286277

Start by locating the EuiListGroup component and reviewing the linked Kibana use case to understand the desired spacing behavior. Trace how list item layout and styling are defined, then verify that the new gutter option creates consistent gaps between elements without changing the default stacked appearance.

Written by the indexing model from the issue text.