[camera_platform_interface] Expand camera platform interface to support pausing the camera preview.

[BeMacized]

This PR adds pausePreview and resumePreview methods to the camera plugin's platform interface, to support the (un)pausing of the camera preview.

Android & iOS implementations for these methods can be found in #4192.

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.

[mvanbeusekom]

LGTM