Feature: #87085 - Fallback options for slug fields

See Issue #87085

Description

In case of SEO optimizations and the daily work of an editor, now it is possible to define a list of fields in the slug configuration as nested array:

The fallback field can also be combined with other fields:

Examples

Configuration value Values of an example page record Resulting slug
['nav_title', 'title'] ['title' => 'Products', 'nav_title' => '', 'subtitle' => ''] /products
['nav_title', 'title'] ['title' => 'Products', 'nav_title' => 'Best products', 'subtitle' => ''] /best-products
['subtitle', 'nav_title', 'title'] ['title' => 'Products', 'nav_title' => 'Best products', 'subtitle' => 'Product subtitle'] /product-subtitle
['nav_title', 'title'], 'subtitle' ['title' => 'Products', 'nav_title' => 'Best products', 'subtitle' => 'Product subtitle'] /best-products/product-subtitle
['seo_title', 'title'], ['nav_title', 'subtitle'] ['title' => 'Products', 'nav_title' => 'Best products', 'subtitle' => 'Product subtitle', 'seo_title' => 'SEO product title'] /seo-product-title/products