[pigeon] FlutterApi error handling (flutter#5008)

* Adds error handling on Flutter Api methods.
* **Breaking Change** Removes `Reply` class from all Java method returns.
* **Breaking Change** Adds `NullableResult` class for all nullable Java method returns.
* **Breaking Change** Nests all enum returns in Objective-c to allow for `nil` response on error.
* **Breaking Change** Renames `Setup` to `SetUp` in Objective-c.

fixes flutter/flutter#118243
fixes flutter/flutter#118245
fixes flutter/flutter#124268
fixes flutter/flutter#135176

[webview_flutter_wkwebview] Fixes bug where `WebkitWebViewController.…

…getUserAgent` was incorrectly returning an empty String (flutter#5062)

It seems that [WKWebView.customUserAgent](https://developer.apple.com/documentation/webkit/wkwebview/1414950-customuseragent?language=objc) is never actually set to nil. It was always an empty String. Even after attempting to set the value to nil, the value would always return as an empty String. This is in contrary to the documentation for the property.

I'm not sure if this is based on the iOS version, so this PR checks if the value is null or is empty.

Fixes flutter/flutter#135813

[webview_flutter] Exposes missing `JavaScriptLogLevel` from platform …

…interface (flutter#5061)

`JavaScriptConsoleMessage` includes a field with type  `JavaScriptLogLevel` and was missing from flutter#4705.

[flutter_markdown] Code decoration (flutter#5060)

Because of using `DecoratedBox` for code block the content is above the decoration. This PR replaces `DecoratedBox` with `Container`.

![image](https://github.com/flutter/packages/assets/1847552/3203ecaf-23d3-44bd-ad73-f1369aa38020)

Issue: flutter/flutter#135858

[webview_flutter] Adds app facing implementation to override console …

…log (flutter#4705)

Adds the app facing implementation for registering a JavaScript console callback. This will allow developers to receive JavaScript console messages in a Dart callback.

This PR contains the `webview_flutter` specific changes from PR flutter#4541.

Fixes flutter/flutter#32908

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

[webview_flutter] Add a method for getting the user agent (flutter#4472)

Aggregate PR for adding a method for retrieving the user agent. 

Internal: b/272582342

[webview_flutter_android][webview_flutter_wkwebview] Adds support for…

… `getUserAgent` for `webview_flutter` platform implementations (flutter#4927)

Platform implementations for flutter#4472

[webview_flutter_android][webview_flutter_wkwebview] Adds support for…

… `getUserAgent` for `webview_flutter` platform implementations (flutter#4927)

Platform implementations for flutter#4472

[webview_flutter_android] Adds Android implementation to override con…

…sole log (flutter#4702)

Adds the Android implementation for registering a JavaScript console callback. This will allow developers to receive JavaScript console messages in a Dart callback.

This PR contains the `webview_flutter_android` specific changes from PR flutter#4541.

Related issue: flutter/flutter#32908

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

[camera] Dispose resources correctly on setDescription (flutter#4003)

setDescription for CameraController was not correctly handling resources. It would recreate the _deviceOrientationSubscription each time it was called, which caused the subscription to not be disposed correctly. It also was not disposing of the previous device camera

*List which issues are fixed by this PR. You must list at least one issue.*

Issue seen here: flutter/flutter#126823