Begin defining Extension-based Notebook Provider (#3172)

Implements provider contribution in the MainThreadNotebook, with matching function calls in the ExtHostNotebook class. This will allow us to proxy through notebook providers (specifically, creation of a notebook manager with required content, server managers) from an extension up through to the main process.

Implemented in this PR:
- Callthroughs for content and server manager APIs
- Very basic unit tests covering provider & manager registration

Not implemented:
- Fuller unit tests on the specific callthrough methods for content & server manager.
- Contribution point needed to test this (so we can actually pass through the extension's existing Notebook implementation)

src/sqltest/utils/testUtils.ts

@@ -0,0 +1,22 @@
+
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+'use strict';
+
+import * as assert from 'assert';
+
+export async function assertThrowsAsync(fn, regExp?: string): Promise<void> {
+    let f = () => {
+        // Empty
+    };
+    try {
+      await fn();
+    } catch (e) {
+      f = () => { throw e; };
+    } finally {
+      assert.throws(f, regExp);
+    }
+}