feat(in_app_messaging): add custom Flutter display for campaigns

[SelaseKay]

Description

Adds an opt-in custom Flutter display path for In-App Messaging. Native SDKs already support replacing the default banner / modal / card / image-only templates; this wires that through to Dart.

• setCustomDisplayEnabled(true) installs the plugin as the native display component. Listening to onMessageDisplay alone does not steal native UI.
• Eligible campaigns are forwarded as InAppMessage (title, body, images, colors, actions, campaign metadata, and data). Apps draw their own widgets and report impress / click / dismiss / reportError so analytics and frequency capping keep working.
• The plugin does not open action URLs.
• Android rebinds the display component on activity resume (the display SDK overwrites it) and restores the default templates when custom display is turned off. iOS sets messageDisplayComponent and reports callbacks through the executor displayDelegate.
• Reuses the Pigeon FlutterApi and campaign/dismiss types from feat(in_app_messaging): expose the message lifecycle events #18584. This PR is stacked on that branch.

Related Issues

• Fixes [firebase_in_app_messaging]: Support custom Flutter UI for In-App Messaging (expose native custom display component) #18381
• Depends on feat(in_app_messaging): expose the message lifecycle events #18584

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]).
This will ensure a smooth and quick review process. Updating the pubspec.yaml and changelogs is not required.

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change.
• No, this is not a breaking change.

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