feat(openapi): Add modified typescript-fetch template

This template is modified from the original one, to handle multiple
status-code handling of response.

Don't ask me how they work, I don't quite understand them in depth...

openapi-template/sagaApiManager.mustache

@@ -0,0 +1,28 @@
+import {
+    Configuration,
+    ConfigurationParameters,
+} from "../index{{importFileExtension}}";
+
+import {
+{{#apiInfo}}
+{{#apis}}
+    {{classFilename}},
+{{/apis}}
+{{/apiInfo}}
+} from "./index{{importFileExtension}}";
+
+export class Api {
+{{#apiInfo}}
+{{#apis}}
+    public static {{#lambda.camelcase}}{{classFilename}}{{/lambda.camelcase}}: {{classFilename}};
+{{/apis}}
+{{/apiInfo}}
+
+    public static init(apiBaseConfig: ConfigurationParameters) {
+{{#apiInfo}}
+{{#apis}}
+        Api.{{#lambda.camelcase}}{{classFilename}}{{/lambda.camelcase}} = new {{classFilename}}(new Configuration(apiBaseConfig));
+{{/apis}}
+{{/apiInfo}}
+    }
+}