Windows platform view API demo

[yaakovschectman]

At least temporarily, this PR serves to demonstrate the planned runner-facing API changes to enable platform views on Windows. In its current state, it will not be merged.

Part of https://github.com/flutter/flutter/issues/138431

Pre-launch Checklist

• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [x] I read the Tree Hygiene wiki page, which explains my responsibilities.
• [x] I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• [x] I listed at least one issue that this PR fixes in the description above.
• [ ] I added new tests to check the change I am making or feature I am adding, or the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• [x] I updated/added relevant documentation (doc comments with ///).
• [ ] I signed the CLA.
• [ ] All existing and new tests are passing.

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