Feat/model backed ui (#1145)

This is an initial PR for a new model-driven UI where extensions can provide definitions of the components & how they're laid out using Containers.
#1140, #1141, #1142, #1143 and #1144 are all tracking additional work needed to improve the initial implementation and fix some issues with the implementation.

Features:
- Supports defining a FlexContainer that maps to a flexbox-based layout.
- Supports creating a card component, which is a key-value pair based control that will lay out simple information to a user. Eventually this will have an optional set of actions associated with it.
- Has a sample project which shows how to use the API and was used for verification

src/sql/workbench/api/common/sqlExtHostTypes.ts

@@ -62,3 +62,24 @@ export enum ScriptOperation {
 	Execute = 5,
 	Alter = 6
 }
+
+export enum ModelComponentTypes {
+	NavContainer,
+	FlexContainer,
+	Card,
+	DashboardWidget,
+	DashboardWebview
+}
+
+export interface IComponentShape {
+	type: ModelComponentTypes;
+	id: string;
+	properties?:  { [key: string]: any };
+	layout?: any;
+	itemConfigs?: IItemConfig[];
+}
+
+export interface IItemConfig {
+	componentShape: IComponentShape;
+	config: any;
+}