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

@@ -411,3 +411,9 @@ export class SqlThemeIcon {
 		this.id = id;
 	}
 }
+
+export interface INotebookManagerDetails {
+	handle: number;
+	hasContentManager: boolean;
+	hasServerManager: boolean;
+}