Adds a XamlUIService property to IReactContext, which exposes ElementFromTag

[acoates-ms]

There is currently no way to get to the XAML element from a reactTag -- which is something requested by several partners.

This creates a XamlUIService class that can be used to expose XAML specific functionality from a ReactContext. This new class exposes ElementFromReactTag which is the equivalent of the viewForReactTag method exposed on iOS. It also exposes DispatchEvent which is currently on ReactContext directly, but is planned to be deprecated from the current class, as we want ReactContext to be platform agnostic, allowing native modules to be sharable across platforms.

Sample usage based on example in the #4327:

    REACT_METHOD(startBringIntoView);
    void startBringIntoView(int reactTag) noexcept {
        auto uiService = winrt::Microsoft::ReactNative::XamlUIService::FromContext(m_reactContext);
        uiService.ElementFromReactTag(reactTag).as<winrt::UIElement>().StartBringIntoView();
    }

Fixes #4327

Microsoft Reviewers: Open in CodeFlow

[asklar]

Thanks for making this WinUI 3 friendly :)

[vmoroz]

[ghost]

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[NickGerleman]

Removing the Backport to 0.62 label, since we need to rename it as part of the deprecation in favor of Request Backport which was already added to this PR. Triage of this item should still happen soon.

[stecrain]

Is this going to be back-ported?

[acoates-ms]

@stecrain - yes it is going to be back-ported.

[Khalef1]

Approved for backport by triage