feat: add upgrade_to_pro_url property

src/Plugin/AbstractPlugin.php

@@ -56,6 +56,13 @@ abstract class AbstractPlugin extends SlimPlugin {
 	 */
 	protected $support_url;
 
+	/**
+	 * Upgrade to pro URL.
+	 *
+	 * @var string
+	 */
+	protected $upgrade_to_pro_url;
+
 	/**
 	 * AbstractPlugin constructor.
 	 *
@@ -216,6 +223,13 @@ abstract class AbstractPlugin extends SlimPlugin {
 			$links        = array_merge( $plugin_links, $links );
 		}
 
+		if ( $this->upgrade_to_pro_url ) {
+			$plugin_links = [
+				'<a href="' . $this->upgrade_to_pro_url . '">' . __( 'Upgrade to PRO', $this->get_text_domain() ) . '</a>',
+			];
+			$links        = array_merge( $plugin_links, $links );
+		}
+
 		return $links;
 	}