[camera] Convert Windows to Pigeon

[stuartmorgan-g]

Replaces all of the manual method channel code in camera_windows with Pigeon.

I attempted to change the structure as little as possible, since this was already a large change. I don't particularly like the way the native result callback objects are managed (passed to Camera and tracked in a map), but I decided that redesigning that would be out of scope and introduced a std::variant to allow minimal changes to that structure. That does slightly undermine the type safety of the callbacks, but it's still strictly enforced at the level of the helpers that interact with the map.

Fixes https://github.com/flutter/flutter/issues/117905

Pre-launch Checklist

• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [x] I read the Tree Hygiene page, which explains my responsibilities.
• [x] I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• [x] I signed the CLA.
• [x] The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• [x] I linked to at least one issue that this PR fixes in the description above.
• [x] I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• [x] I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or this PR is exempt from CHANGELOG changes.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] I added new tests to check the change I am making, or this PR is test-exempt.
• [x] All existing and new tests are passing.

[stuartmorgan-g]

@cbracken Ping on this review in case you missed the notification while you were out.

[cbracken]

Back as of yesterday and mostly caught up on mail. Looking!