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/parts/modelComponents/components.contribution.ts

@@ -0,0 +1,15 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the Source EULA. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+import FlexContainer from './flexContainer.component';
+import CardComponent from './card.component';
+import { registerComponentType } from 'sql/platform/dashboard/common/modelComponentRegistry';
+import { ModelComponentTypes } from 'sql/workbench/api/common/sqlExtHostTypes';
+
+export const FLEX_CONTAINER = 'flex-container';
+registerComponentType(FLEX_CONTAINER, ModelComponentTypes.FlexContainer, FlexContainer);
+
+export const CARD_COMPONENT = 'card-component';
+registerComponentType(CARD_COMPONENT, ModelComponentTypes.Card, CardComponent);