Astro adapter

[Yuangwang]

Adds support for Astro in Firebase App Hosting.

To use follow these instructions:

1. Install the @apphosting/astro-adapter Astro adapter with the following command in your terminal:
   npx astro add @apphosting/astro-adapter
2. Add the adapter with the standalone rendering mode to your astro.config.* file:

import { defineConfig } from 'astro/config';
import node from '@apphosting/astro-adapter';

export default defineConfig({
  output: 'server',
  adapter: node({
    mode: 'standalone',
  }),
});

3. Push the changes to github and kickoff a new rollout