[iOS] Support gradient colors for selected item in TabBarView

[mischreiber]

Platforms Impacted

• iOS
• visionOS
• macOS

Description of changes

Added a new API to TabBarView, selectedItemGradient. This is an optional property of type CAGradientLayer? that, when set, will override the tab bar view's tint color. Instead, this gradient will be resized and used as a mask, giving the selected item a gradient fill.

Additionally, when this is set, the selected item's titleLabel will not receive any tinting.

Binary change

TBD

Verification

Verified the tint on iPhone and iPad, dark and light modes.

Visual Verification

Pull request checklist

This PR has considered:

• Light and Dark appearances
• iOS supported versions (all major versions greater than or equal current target deployment version)
• VoiceOver and Keyboard Accessibility
• Internationalization and Right to Left layouts
• Different resolutions (1x, 2x, 3x)
• Size classes and window sizes (iPhone vs iPad, notched devices, multitasking, different window sizes, etc)
• iPad Pointer interaction
• SwiftUI consumption (validation or new demo scenarios needed)
• Objective-C exposure (provide it only if needed)

Microsoft Reviewers: Open in CodeFlow