Add highlight styling to list style (#1919)

# Summary

Added the ability to customize highlighted (tapped state) background
color of List Items. Previously, it was locked to the default OS value.

# What problem you’re solving

Unable to change the highlighted background color of list items.

# Your approach to fixing the problem

Added a style property to `ListItemStyle` - as an optional value,
because the default value is OS protected and can only be approximated.
This also means it won't result in any changes to existing
implementations.

# Any tests you wrote

As a highlighted state that only occurs on tapping, I did not see a way
to integrate with snapshot tests. Happy to add some if someone can point
me in the right direction.

# Release notes
<changed>
You can now highlight a list item by changing its background color when
the shopper selects it. To do this, use the new optional
`ListItemStyle.highlightedBackgroundColor` property.
</changed>

Adyen/UI/List/ListCell.swift

@@ -24,6 +24,16 @@ public final class ListCell: UITableViewCell {
         fatalError("init(coder:) has not been implemented")
     }
 
+    override public func setHighlighted(_ highlighted: Bool, animated: Bool) {
+        super.setHighlighted(highlighted, animated: animated)
+
+        guard let highlightedBackgroundColor = item?.style.highlightedBackgroundColor else {
+            return
+        }
+
+        contentView.backgroundColor = highlighted ? highlightedBackgroundColor : item?.style.backgroundColor
+    }
+
     // MARK: - Item
 
     /// The item displayed in the cell cell.

Adyen/UI/List/ListItemStyle.swift

@@ -42,6 +42,9 @@ public struct ListItemStyle: ViewStyle {
 
     public var backgroundColor = UIColor.Adyen.componentBackground
 
+    /// Background color when highlighted (tapped).
+    public var highlightedBackgroundColor: UIColor?
+
     /// Initializes the list item style.
     ///
     /// - Parameter title: The title style.