Port the Azure Resource Explorer extension to core. (#2701)

* Port the Azure Resource Explorer extension to core.

This will enable Azure viewlet by default in the next release.

- Moving this code from the SQL Server 2019 extension to Azure Data Studio core
- Ported tests and verified they work in the integration tests.sh file
- Fixed an issue that caused integration tests to fail if you have a SQL Server 2019 big data cluster endpoint listed, but the extension isn't installed.

extensions/azurecore/src/constants.ts

@@ -0,0 +1,8 @@
+'use strict';
+
+export const extensionConfigSectionName = 'azure';
+export const ViewType = 'view';
+
+export enum BuiltInCommands {
+	SetContext = 'setContext'
+}