feat(rest-api-client): add the method of app.addPlugins

[shabaraba]

Why

• Support app.addPlugins method to be able to add Plug-ins to an App.

What

• Add a document for the new methods.
• Add new unit tests.

How to test

1. prepare installed plug-ins and apps in Kintone.
2. run below code and check logs:

import {KintoneRestAPIClient} from '@kintone/rest-api-client';

const client = new KintoneRestAPIClient();

const pluginId = (await client.app.getPlugins({app: 1})).plugins[0].id;

console.log(await client.app.getPlugins({app: 2, preview: true}));
console.log(await client.app.addPlugins({app: 2, ids: [pluginId]}));
console.log(await client.app.getPlugins({app: 2, preview: true}));

Checklist

• Read CONTRIBUTING.md
• Updated documentation if it is required.
• Added tests if it is required.
• Passed pnpm lint and pnpm test on the root directory.

[tasshi-me]

LGTM!