Add GetConnectionString command (without build break) (#2120)

* Revert "Revert "Adds "Get Connection String" command (#2108)" (#2116)"

This reverts commit c6d1fa2b7d.

* Fix build breaks

src/sqltest/stubs/connectionManagementService.test.ts

@@ -253,4 +253,8 @@ export class TestConnectionManagementService implements IConnectionManagementSer
 	getActiveConnectionCredentials(profileId: string): { [name: string]: string } {
 		return undefined;
 	}
+
+	getConnectionString(ownerUri: string): Thenable<string> {
+		return undefined;
+	}
 }