Exposing accessibility prop for access keys

Summary

Add a prop to set the access key UIA property on a component.

Motivation

RN Win32 is working on a feature which requires exposing a prop to add access key accessibility information to a component. Guidance from https://github.com/microsoft/react-native-windows/issues/3450 mentions using accessibilityHint to set the property, but we've found that accessibilityHint doesn't consistently get explicitly announced by Narrator for certain control types like MenuItems (see here). So, we will be going with exposing a prop to set the UIA AccessKey property.

Opening this issue to revisit this guidance for RNW and discuss if proper support for access key accessibility should be added. It would be good to have consistency across platforms for exposing accessibility information.

Basic Example

<View focusable accessible onKeyDown={handleAccessKey()} accessibilityAccessKey={'A'}>
    <Text>Test</Text>
</View>

Open Questions

Should accelerator key accessibility also be considered?

Start by reviewing the RNW guidance in issue 3450 and the linked Narrator behavior reference, then trace how accessibility props are exposed to Windows UIA components. Done means a documented prop consistently sets the UIA AccessKey property across relevant control types, with the accelerator-key question resolved.