Allow customizing APIs in WorkspaceClient/AccountClient (#213)

## Changes
Feature request from @satviksr-db, allowing him to provide custom
implementations of the top-level API. This mirrors what we've done in
the Go SDK.

## Tests
No tests needed, as this isn't a behavior change.

Author: mgyucht

.codegen/account.java.tmpl

@@ -45,9 +45,14 @@ public class AccountClient {
   }
   {{end}}{{end}}
   {{range .Services}}{{if .IsAccounts}}
-  /** Override {{.PascalName}}API with mock */
+  /** Replace the default {{.PascalName}}Service with a custom implementation. */
   public AccountClient with{{(.TrimPrefix "account").PascalName}}Impl({{.PascalName}}Service {{.CamelName}}) {
-    {{(.TrimPrefix "account").CamelName}}API = new {{.PascalName}}API({{.CamelName}});
+    return this.with{{(.TrimPrefix "account").PascalName}}API(new {{.PascalName}}API({{.CamelName}}));
+  }
+
+  /** Replace the default {{.PascalName}}API with a custom implementation. */
+  public AccountClient with{{(.TrimPrefix "account").PascalName}}API({{.PascalName}}API {{.CamelName}}) {
+    this.{{(.TrimPrefix "account").CamelName}}API = {{.CamelName}};
     return this;
   }
   {{end}}{{end}}

.codegen/workspace.java.tmpl

@@ -63,9 +63,14 @@ public class WorkspaceClient {
   }
   {{end}}{{end}}
   {{range .Services}}{{if not .IsAccounts}}
-  /** Replace {{.PascalName}}API implementation with mock */
+  /** Replace the default {{.PascalName}}Service with a custom implementation. */
   public WorkspaceClient with{{.PascalName}}Impl({{.PascalName}}Service {{.CamelName}}) {
-    {{.CamelName}}API = new {{template "api" .}}({{.CamelName}});
+    return this.with{{.PascalName}}API(new {{template "api" .}}({{.CamelName}}));
+  }
+
+  /** Replace the default {{.PascalName}}API with a custom implementation. */
+  public WorkspaceClient with{{.PascalName}}API({{template "api" .}} {{.CamelName}}) {
+    this.{{.CamelName}}API = {{.CamelName}};
     return this;
   }
   {{end}}{{end}}