[AND-11] Expose flagUser/unflagUser on MessageListController and MessageListViewModels.

[VelikovPetar]

🎯 Goal

Exposes the flagUser() and unflagUser() methods on the MessageListController and MessageListViewModel (xml and compose) so that they can be invoked from the UI components, without accessing the ChatClient directly.

Linear: https://linear.app/stream/issue/AND-11/expose-flaguserunflaguser-on-messagelistcontrollermessagelistviewmodel

🛠 Implementation details

• Expose flagUser()/unflagUser() on the MessageListController (delegates the call to the ChatClient)
• Create new ErrorEvent.FlagUserError and ErrorEvent.UnflagUserError for handling errors which occurred while performing the corresponding operation
• Expose flagUser()/unflagUser() on the MessageListViewModel(compose)
• Create new Event.FlagUser and Event.UnflagUser for triggering the corresponding operations from the xml SDK.

🎨 UI Changes

NA

🧪 Testing

• Not really testable, as we don't provide a UI for the flagUser operations.

☑️Contributor Checklist

General

• I have signed the Stream CLA (required)
• Assigned a person / code owner group (required)
• Thread with the PR link started in a respective Slack channel (#android-chat-core or #android-chat-ui) (required)
• PR targets the develop branch
• PR is linked to the GitHub issue it resolves

Code & documentation

• Changelog is updated with client-facing changes
• New code is covered by unit tests
• Comparison screenshots added for visual changes
• Affected documentation updated (KDocs, docusaurus, tutorial)

☑️Reviewer Checklist

• UI Components sample runs & works
• Compose sample runs & works
• UI Changes correct (before & after images)
• Bugs validated (bugfixes)
• New feature tested and works
• Release notes and docs clearly describe changes
• All code we touched has new or updated KDocs

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