Closes #18047.

Adds a new REST API endpoint to grab details from a external URL. So now if I want to get the contents of the title tag on https://wordpress.org I can send a request to this API endpoint and it will return the value in the response for use in the Gutenberg UI.

Why might this be useful? See #19387.

As part of #17846 we need to display the contents of the <title> tag from the url the user has typed into the input field. This has now long since shipped. Nonetheless, it might be a nice enhancement to be able to show details about the remote URL that has been selected.

This PR addresses this by adding a new experimental custom Gutenberg REST API endpoint. This endpoint:

• makes a request for a remote URL.
• parses the response in a DOMDocument we should investigate this in future (see here)
• retrieves the title tag contents.

To prevent abuse, the endpoint is authenticated and requires the user to have the edit_posts capability to be able to make a request.

How has this been tested?

• Manual testing (see below)
• Automated tests

Automated Testing Instructions

npm run test-php should run the unit tests which reside in phpunit/class-wp-rest-url-details-controller-test.php.

Manual Testing Instructions

• Check out this PR.
• Open the Gutenberg local testing env at http://localhost:8889/wp-admin/ and login.
• Create a new Post.
• Choose one of the following options:

Option 1

• Open devtools and in the console enter:

wp.apiFetch( { path: '/__experimental/url-details/?url=https://wordpress.org' } ).then( data => {
    console.log( data );
} );

You'll see the response returned there.

Option 2

• Open devtools and in the console enter wpApiSettings.nonce. You should see a valid nonce returned as a string.
• Copy the nonce.
• In your browser goto: http://localhost:8889/wp-json/__experimental/url-details/?url=https://wordpress.org&_wpnonce=%YOUR_NONCE_HERE% - be sure to replace the nonce value with that copied from the previous step.
• You should see a valid REST API response containing the contents of the title tag from wordpress.org (ie: "Blog Tool, Publishing Platform, and CMS \u2014 WordPress.org").

Types of changes

New feature (non-breaking change which adds functionality).

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.