Allow build from 'fastify-cli/helper.js' to accept an existing module object

[segevfiner]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the feature has not already been requested

🚀 Feature Proposal

Allow build from 'fastify-cli/helper.js' to accept an existing module object instead of importing/requiring by itself.

Motivation

vitest doesn't support fully dynamic imports that well, especially inside imported modules which default to be externalized, which means the import will happen using Node's standard require/import which can't import TypeScript and has other limitations

Example

import appPlugin from '../src/app';

const app = await helper.buildApp(appPlugin, ['--other-arg'], await config());