Support config mult cdn path

[peng]

Body

Summary

I want to use different cdn url prefix with static file, like image, js, css.. But astro now not support.

Background & Motivation

I want to config different cdn url prefix with static file in my project. But not support.

So my project image manual upload, css configured as build.inlineStylesheets = 'always', only js files use the configured cdn address.

We have discussed it in this issues: withastro/astro#9700 (comment)

Goals

A concise, bulleted-list outlining the intended goals of this RFC.

• What are the exact problems that you are trying to solve with this RFC?

Support multiple cdn url prefix for static file.

• If this RFC isn't approved, these goals can be used to develop alternative solutions.

I only use my current config, image manual upload, css configured as build.inlineStylesheets = 'always', only js files use the configured cdn address.

Example

If the proposal involves a new or changed API, then include a basic code example.
Otherwise, omit this section if it's not applicable.

Use build. assetsPrefix option, current assetsPrefix is string type. Add function type.

// assetsPrefix function

import { defineConfig } from 'astro/config';

export default defineConfig({
  build: {
    assetsPrefix: (fileType) => {
      if (fileType === 'js') {
        return 'https://js.example.com/'
      } else if (fileType === 'css') {
        return 'https://css.example.com/'
      }

      return ''
    }
  }
});

fileType type is string, value is file extension. Function return is string type.

[yhnsw0711]

We also need this feature, and this configuration form is also good.

[bluwy]

Completed in withastro/astro#10189