[camera] Add Preview related methods to CameraX Plugin

[camsim99]

One primary use case of the camera plugin is to present a preview of the shot the camera will take.

The following is the list of methods in the Flutter camera plugin API that will need to be implemented to enable this use case and match the behavior of the camera_android plugin:

• pausePreview()
• resumePreview()
• startPreviewWithImageStream()
• startPreview()

[github-actions]

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