Add accessibilityHint for iOS (#18093)

Summary:
This adds the accessibilityHint for View, Text and Touchable* on iOS.
The accessibilityHint provides some more information about an element
when the accessibilityLabel is not enough.

The accessibilityHint is a core accessibility property on iOS.

From https://developer.apple.com/documentation/objectivec/nsobject/1615093-accessibilityhint:
> An accessibility hint helps users understand what will happen when they perform an action on the accessibility element when that result is not obvious from the accessibility label.

Related issue: https://github.com/facebook/react-native/issues/14706

The npm scripts `test`, `flow`, `lint` and `prettier` are satisfied.

I added a couple of examples to the RNTester app. The Accessibility Inspector on Mac helps debugging accessibility stuff on a simulator, but it does not show the accessibilityHint. Therefore I tested the RNTester app on an iPhone 8 device using VoiceOver to verify the hint functionality. It works fine, and I've tested disabling and enabling "read hints" in the VoiceOver settings on the phone.

https://github.com/facebook/react-native-website/pull/222

[IOS][FEATURE][Accessibility] - Add accessibilityHint for View, Text, Touchable* on iOS
Closes https://github.com/facebook/react-native/pull/18093

Reviewed By: hramos

Differential Revision: D7230780

Pulled By: ziqichen6

fbshipit-source-id: 172ad28dc9ae2b67ea256100f6acb939f2466d0b

Libraries/Components/Touchable/TouchableOpacity.js

@@ -256,6 +256,7 @@ const TouchableOpacity = ((createReactClass({
       <Animated.View
         accessible={this.props.accessible !== false}
         accessibilityLabel={this.props.accessibilityLabel}
+        accessibilityHint={this.props.accessibilityHint}
         accessibilityComponentType={this.props.accessibilityComponentType}
         accessibilityTraits={this.props.accessibilityTraits}
         style={[this.props.style, {opacity: this.state.anim}]}