[camera] Dart Interface for camera refactor

[bparrishMines]

Description

Beginning of refactor of the camera plugin. This only implements the camera interface and shared classes (e.g. camera mixins, CameraChannel & CameraTesting )

This also adds the publish_to: none to the pubspec.yaml until the refactor is finished.

Related Issues

flutter/flutter#31225

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (flutter analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate a breaking change in CHANGELOG.md and increment major revision).
• No, this is not a breaking change.

[mklim]

LGTM

nit: Can you add a quick README to /new saying that this is a work in progress, and link to the issue tracking the refactor?

[mklim]

Thanks for clarifying!

Discussed offline, I'm thinking this is kind of a foot gun. Sounds like the current plugin behavior is to guard against this by disposing of multiple instances under the hood. I think we should probably maintain it here, but that doesn't need to block this PR.