[camera] Fix coordinate rotation for setting focus- and exposure points on iOS

[BeMacized]

As of right now, the setFocusPoint and setExposurePoint methods in the camera plugin do not correctly rotate the supplied coordinates according to the UI orientation, causing them to only show correct behaviour in one orientation.

This PR fixes this issue on iOS.
The fix for Android has been included in the part 9 PR of the android refactor (#4059).

Relevant issue:

• [camera] Coordinates set by setFocusPoint and setExposurePoint are only valid in a single orientation. flutter#86468

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

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.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[mvanbeusekom]

LGTM % some nits (most important is the text for the CHANGELOG.md)

[mvanbeusekom]

LGTM