Adds support for video stabilization to camera_platform_interface, camera_avfoundation, camera_android_camerax and camera packages.

The video stabilization modes are defined in the new VideoStabilizationMode enum defined in camera_platform_interface:
```dart
/// The possible video stabilization modes that can be capturing video.
enum VideoStabilizationMode {
  /// Video stabilization is disabled.
  off,

  /// Basic video stabilization is enabled.
  /// Maps to CONTROL_VIDEO_STABILIZATION_MODE_ON on Android
  /// and throws CameraException on iOS.
  on,

  /// Standard video stabilization is enabled.
  /// Maps to CONTROL_VIDEO_STABILIZATION_MODE_PREVIEW_STABILIZATION on Android
  /// (camera_android_camerax) and to AVCaptureVideoStabilizationModeStandard
  /// on iOS.
  standard,

  /// Cinematic video stabilization is enabled.
  /// Maps to CONTROL_VIDEO_STABILIZATION_MODE_PREVIEW_STABILIZATION on Android
  /// (camera_android_camerax) and to AVCaptureVideoStabilizationModeCinematic
  /// on iOS.
  cinematic,

  /// Extended cinematic video stabilization is enabled.
  /// Maps to AVCaptureVideoStabilizationModeCinematicExtended on iOS and
  /// throws CameraException on Android.
  cinematicExtended,
}
```
There is some subjectivity on the way with which I mapped the modes to both platforms, and here's a document that compares the several modes: https://docs.google.com/spreadsheets/d/1TLOLZHR5AcyPlr-y75aN-DbR0ssZLJjpV_OAJkRC1FI/edit?usp=sharing, which you can comment on.

*List which issues are fixed by this PR. You must list at least one issue.*
Partially implements flutter/flutter#89525

Replaces direct event channel usage with type-safe Pigeon event channels now that the plugin has been migrated to Swift (Pigeon doesn't have event channel support in the Obj-C generator, so this had been blocked).

Also includes opportunistic cleanup of conditional import of `camera_avfoundation_objc`, since I noticed it in the files I was working in; this is dead code now that the Swift migration is complete, because the `camera_avfoundation_objc` no longer exists.

Fixes flutter/flutter#182542

## Pre-Review Checklist

[^1]: Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling.

Replaces the two checklist entries for version and CHANGELOG, whose wording was specific to continuous release, to a single more generic line that links to the instructions, so that it encompasses both continuous and batched releases.

Removes deprecated usesCleartextTraffic. 

Addresses: #182553

## Pre-Review Checklist

…#11113)

Tidy up some contributing text, and note tools needed for formatting.

* The "state diagram" paragraph was a really long sentence which changed tense halfway through.
* `dart:mirrors` is a library.

## Pre-Review Checklist

Removes deprecated usesCleartextTraffic. 

Addresses: #182553

## Pre-Review Checklist

Moves the platform-specific setup details from the main README to the implementation package READMEs, per our current best practice.

Minor cleanup as part of moving things:
- Simplifies iOS description and adds a link to the Apple docs.
- Removes the Java variant of the app setup instructions; new apps are overwhelmingly Kotlin, and Java users can extrapolate from the Kotlin.
- Removes an obsolete section about detecting face ID support on Android; we no longer support specific auth method detection on Android at all.

Part of flutter/flutter#182772

## Pre-Review Checklist

[^1]: Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling.

The linked issues here was closed to further feedback, removing as outdated.

Comment: flutter/flutter#90218 (comment)

> Thanks for all the feedback on this issue. I'm going to close this issue for now, but feel free to keep the feedback coming in new issues or on Discord.

## Pre-Review Checklist

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[^1]: Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling.