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/services/model/modelViewService.ts

@@ -0,0 +1,22 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the Source EULA. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+'use strict';
+import * as sqlops from 'sqlops';
+import { IItemConfig, IComponentShape } from 'sql/workbench/api/common/sqlExtHostTypes';
+
+export interface IView {
+	readonly id: string;
+	readonly connection: sqlops.connection.Connection;
+	readonly serverInfo: sqlops.ServerInfo;
+}
+
+export interface IModelView extends IView {
+	initializeModel(rootComponent: IComponentShape): void;
+	clearContainer(componentId: string): void;
+	addToContainer(containerId: string, item: IItemConfig): void;
+	setLayout(componentId: string, layout: any): void;
+	setProperties(componentId: string, properties: { [key: string]: any }): void;
+}