Add prepare route for MyYoast connection

[herregroen]

We want to have a new admin route to initiate the connection to MyYoast.

This route will be called from our onboarding wizard our premium plugin page and should do the following:

• Ensure the current user has sufficient privileges to make the connection ( can manage_options ).
• Generate a UUID ( wp_generate_uuid4() ).
• Store the UUID ( see Create OAuth Client Class #12093 ).
• Redirect the user to https://my.yoast.com/connect with the following parameters:
  • url: the home_url or network_home_url as is currently used for licensing.
  • client_id: the generated UUID.
  • extensions: an array of the active Yoast extensions by slug ( premium, local etc. ).
  • redirect_url: The url of the route implemented in ( Add callback route for MyYoast connection #12135 ).

Test instructions:
Free & single:

• Have only free enabled on a non-network site.
• Visit the prepare route.
• The redirect should have your home_url as the url and an empty extensions array.

Free & multi:

• Have only free enabled on a single site on a network.
• Visit the prepare route.
• The redirect should have your home_url as the url and an empty extensions array.

Free & multi network:

• Have only free enabled network-enabled.
• Visit the prepare route.
• The redirect should have your home_url as the url and an empty extensions array.

Premium & multi:

• Have only premium enabled on a single site on a network.
• Visit the prepare route.
• The redirect should have your home_url as the url and the extensions array should contain a single element which is the slug of premium.

Premium, addon & multi:

• Have premium and one of the addons enabled on a single site on a network.
• Visit the prepare route.
• The redirect should have your home_url as the url and the extensions array should contain two elements which are the slug of premium and the slug of your chosen addon.

parent #12136
Needs #12093