[EuiSearchBar] Allow custom filter component to be rendered

[sebelga]

As part of the work the improve UX for the <TableListView /> in Kibana (the page rendering the list of dashboards, visualisations, maps) we want to be able to render a custom panel when clicking on a filter button next to the search bar.

This panel needs to have a handler to update the <EuiSearchBar /> component and its filters configuration to allow that.

Proposal

The consumer could render the filter with a configuration like this one

// new filter configuration
{
  type: 'custom_panel',
  name: 'Tags',
  contentRender: ({ setFilters }) => {
     return <MyCustomPanel onChange={setFilters} />
  },
}

Design of the tag filtering we want to implement

[sebelga]

@chandlerprall I will probably start the work on this in the coming weeks. Can you let me know if this seems ok to you and if you have any concern? Cheers

[chandlerprall]

We synced over slack, the goal is implement over the next week and include it in EUI's 8.5 backport