Implement iOS VoiceOver support

[IsaMorphic]

What does the pull request do?

This is a twin PR to #17704 that implements support for VoiceOver on iOS devices. Importantly, the UIAccessibility & UIAccessibilityContainer informal protocols are leveraged to create cohesion between Avalonia's automation system and that of the iOS ecosystem.

What is the current behavior?

Currently, Avalonia apps targeting the iOS platform do not expose user controls to the operating system's accessibility pipeline. This makes it impossible for blind and visually impaired users to use those apps with a screen reader or braille display.

What is the updated/expected behavior with this PR?

This PR enables users reliant on VoiceOver features to properly navigate Avalonia's view heirarchy using the appropriate, well-known shortcuts and gestures.

How was the solution implemented (if it's not obvious)?

The PR implements support for VoiceOver via Avalonia's AutomationPeer API by wrapping those instances in a subclass of UIAccessibilityElement that implements all the necessary formal and informal protocols to create a fully realized experience for low vision users.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0054393-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0054403-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0054407-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0054449-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0054459-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0054504-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]