[EuiInMemoryTable] Add the ability to inject arbitrary content between the searchbar and the table

[pgayvallet]

EuiInMemoryTable does not allow to inject content between the search bar and the table components.

In some cases, consumers may need to add an additional section of controls in-between

Adding an additional property to the EuiInMemoryTable props to allow to prepend arbitrary react elements before the table, like it is commonly done for many EUI components, such as toolsLeft / toolsRight for EuiSearchBar or labelAppend for EuiFormRow would answer this need.

related code: https://github.com/elastic/eui/blob/master/src/components/basic_table/in_memory_table.tsx#L678-L683

Context: for Kibana's SavedObjects tagging feature, it is expected to have the number of selected items and the batch actions between the search bar and the table. Implementing this would allow us to be able to use EuiInMemoryTable for that instead of reimplementing most of its features just to separate the searchbar from the table.

cc @ryankeairns

[ryankeairns]

I think this is probably the same as: #4010

[pgayvallet]

#4010 is a full spec of an utility bar that could be added to EUI tables and datagrids (and is, imho, unlikely to be done for 7.11). This issue is less specific by allowing to inject custom user content between the searchbar and the table (which would be sufficient atm for our needs)