[camera] Add availableCameras() to CameraX plugin

[camsim99]

Proof of concept for https://github.com/flutter/flutter/issues/107260. Adds the availableCameras() method to the CameraX plugin.

Pre-launch Checklist

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

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