Add nextImageExportOptimizer_assetPrefix

[ducdigital]

This pull request add a setting in next.config.js to allow serving image from CDN

// next.config.js
{ "env": {
"nextImageExportOptimizer_assetPrefix": "https://cdn.yourhost.com/"
}}

It simply append the CDN url to generated image url. The deployment to any cdn should be handle separately.

Apologize for some line space changes due to my editor's linter.

Tested on next@16.1.6