[Internal] Move templates in the code generator (#1075)

## Changes
<!-- Summary of your changes that are easy to understand -->
Move templates in the code generator.

## Tests
<!-- 
How is this tested? Please see the checklist below and also describe any
other relevant tests
-->
The generator is working correctly with the new configuration.

.codegen.json

@@ -1,25 +1,6 @@
 {
-  "formatter": "go fmt ./... && go run golang.org/x/tools/cmd/goimports@latest -w $FILENAMES",
+  "mode": "go_v0",
   "changelog_config": ".codegen/changelog_config.yml",
-  "packages": {
-    ".codegen/api.go.tmpl": "service/{{.Name}}/api.go",
-    ".codegen/impl.go.tmpl": "service/{{.Name}}/impl.go",
-    ".codegen/interface.go.tmpl": "service/{{.Name}}/interface.go",
-    ".codegen/model.go.tmpl": "service/{{.Name}}/model.go"
-  },
-  "batch": {
-    ".codegen/workspaces.go.tmpl": "workspace_client.go",
-    ".codegen/accounts.go.tmpl": "account_client.go",
-    ".codegen/service-pkg.go.tmpl": "service/pkg.go",
-    ".codegen/error_mapping.go.tmpl": "apierr/error_mapping.go",
-    ".codegen/error_overrides.go.tmpl": "apierr/error_overrides.go",
-    ".codegen/error_alias.go.tmpl": "error_alias.go",
-    ".codegen/mock_workspace_client.go.tmpl": "experimental/mocks/mock_workspace_client.go",
-    ".codegen/mock_account_client.go.tmpl": "experimental/mocks/mock_account_client.go"
-  },
-  "examples": {
-    ".codegen/examples_test.go.tmpl": "service/{{.Package}}/{{.SnakeName}}_usage_test.go"
-  },
   "version": {
     "version/version.go": "const Version = \"$VERSION\""
   },
@@ -28,6 +9,7 @@
       "go"
     ],
     "post_generate": [
+      "make fmt",
       "go run github.com/vektra/mockery/v2@0229bd4bb4357cd09af7ac15ecab85022d296b12"
     ]
   }