[PECO-1020] Adding mock constructor method for WorkspaceClient to support mocked ApiClient (#144)

Adding additional constructor in WorkspaceClient to support mocking
ApiClient

## Changes
A new constructor in WorkspaceClient to support mocking ApiClient

## Tests
Ran $mvn clean package to verify all test cases and compilation errors

---------

Signed-off-by: Gopal Lal <gopal.lal@databricks.com>

Author: gopalldb

.codegen/workspace.java.tmpl

@@ -46,8 +46,13 @@ public class WorkspaceClient {
 
   /** Constructor for mocks */
   public WorkspaceClient(boolean mock) {
-    apiClient = null;
-    config = null;
+    this(mock, null /* apiClient */);
+  }
+
+  /** Constructor for mocks */
+  public WorkspaceClient(boolean mock, ApiClient apiClient) {
+    this.apiClient = apiClient;
+    this.config = null;
   }
   {{range .Services}}{{if not .IsAccounts}}
   {{if .Description}}/**