Add consumer group container tree item model

[shouples]

For use in the (currently-named) Topics view, similar to how we handle containers in the Flink Database view:

vscode/src/models/flinkDatabaseResourceContainer.ts

Lines 27 to 28 in 48c3dba

	/** A container {@link TreeItem} for resources to display in the Flink Database view. */
	export class FlinkDatabaseResourceContainer<T extends FlinkDatabaseResource | FlinkArtifact>

vscode/src/viewProviders/flinkDatabase.ts

Lines 38 to 44 in 48c3dba

	// top-level container tree items with context values for attaching commands
	const RELATIONS_CONTAINER = new FlinkDatabaseResourceContainer<FlinkRelation>(
	FlinkDatabaseContainerLabel.RELATIONS,
	[],
	"flink-database-relations-container",
	new ThemeIcon(IconNames.TOPIC),
	);