[Logs Explorer] Add support for virtual columns in the data table

[gbamparop]

📓 Summary

Virtual columns will be introduced in Logs Explorer whose values are created on the fly and are not part of the mapping, aiming to provide useful information to users which is easier to consume.

As part of this work, Discover will be updated to support the virtual columns in the data table.

Related to #169874.

✔️ Acceptance criteria

• Should update the Discover state when the virtual columns are added
• Should enable consumers to add icons and tooltips on the column name
• Should enable consumers to add a custom React component in the cell value

💡 Implementation hints

Inject the columns into the Discover state

The source of truth for the columns displayed in the document table is the column property on the Discovper app state. For every entry (except for _source) the grid will try to render a column, regardless of whether it's part of the search hit or not. The particular rendering is delegated to the renderer as described in the next section. The log explorer already sets the default columns upon initialization, so this should work analogously.

Define an external custom renderer

The unified data grid used in Discover has a default renderer for all cells, which applies field formatter overrides defined in the data view and falls back to field-type-specific formatters otherwise. In addition, the unified data grid has a property externalCustomRenderers using which cell renderer overrides can be defined programmatically for columns of a certain name. It is currently not yet exposed via a customization point in the main Discover component, but once it is the Log Explorer could pass renderers that match the names of the virtual columns injected above.

The renderer is responsible for both the "cell representation", which should be optimized for space efficiency as well as the "popover view", which offers room for more elaborate representations. The two cases are distinguished via the isDetails property.

[elasticmachine]

Pinging @elastic/obs-ux-logs-team (Team:obs-ux-logs)

[achyutjhunjhunwala]

Example of Custom Renderer used by the Cloud Security Team-

kibana/x-pack/plugins/cloud_security_posture/public/pages/configurations/latest_findings/latest_findings_table.tsx

Lines 71 to 82 in 0d17a94

	const customCellRenderer = (rows: DataTableRecord[]) => ({
	'result.evaluation': ({ rowIndex }: EuiDataGridCellValueElementProps) => (
	<CspFindingRenderer row={rows[rowIndex]}>
	{({ finding }) => <CspEvaluationBadge type={finding.result.evaluation} />}
	</CspFindingRenderer>
	),
	'@timestamp': ({ rowIndex }: EuiDataGridCellValueElementProps) => (
	<CspFindingRenderer row={rows[rowIndex]}>
	{({ finding }) => <TimestampTableCell timestamp={finding['@timestamp']} />}
	</CspFindingRenderer>
	),
	});