Windows: Add CoreWindow implementation of FlutterWindowsView (Flutter#70205)

[clarkezone]

Description

This change adds a CoreWindow implementation of FlutterWindowView. The UWP target does not yet light up end-to-end with this change.

Related Issues

flutter/flutter#70196
flutter/flutter#14967

Tests

I added the following tests:

There is currently no test infrastructure in place for running tests on Windows in AppContainer mode. That will either be added in future commits to this PR or in a future PR

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Breaking Change

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[clarkezone]

Feedback complete.

[stuartmorgan-g]

Just nits; looks like we're in good shape for the first step. LGTM with the comments addressed!

To capture from discussion above for posterity: I do think we'll need to rework some things to reduce ifdefing, especially around object constructors, but I think we can iterate on that over time, and where it would affect public API, come up with a migration plan that's more comprehensive when we know more about how the rollout of the UWP option will look.