[camera] Add Android & iOS implementations for pausing the camera preview

[BeMacized]

This PR implements the pausePreview and resumePreview interface methods from the camera plugin's platform interface (added in #4191), to support the (un)pausing of the camera preview.

As this PR depends on changes made by the Android refactor (#4059), this PR will remain in draft state until those changes have been merged.

Related issues:

• [camera] pausePreview and resumePreview flutter#34997
• Camera: Missing "freeze"(stopPreview) functionality flutter#27079

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. (Note that 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.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

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[nt4f04uNd]

I wonder if my old PR #3531 should be closed because pausing the preview natively seems more reasonable

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[BeMacized]

Closing in favour of #4258