[EuiInMemoryTable] add the ability to control the spacer's size when using `childrenBetween`

[pgayvallet]

#4103 Introduced a new childrenBetween property to EuiInMemoryTable to inject arbitrary content between the search bar and the actual table. However currently, using the childrenBetween property automatically adds a spacer between the content and the table, without letting the developer have any control over it:

https://github.com/elastic/eui/blob/master/src/components/basic_table/in_memory_table.tsx#L695

In some designs, such as the SO tagging management screen in Kibana, we need to get rid of this spacer to have the custom content directly on top of the table:

A possible solution would be to add a childrenBetweenSpacerSize property. After a discussion with @cchaos, it seems that just removing this spacer altogether and relying on the developer to add it manually when they need to do so would also makes sense.

[cchaos]

I think we can easily just remove this spacer altogether. It will rely on consumers to then add their own, but is an easier solution to customize and maintain.