[PBE-5701] Expose top and bottom bar in MessageScreen

[aleksandar-apostolov]

🎯 Goal

Expose the bottom and top bar of the Scaffold inside the MessageScreen.

🛠 Implementation details

The Scaffold topBar contains the MessageListHeader and the bottomBar contains the MessageComposer instead of exposing the list header or the composer, we just expose the bottom bar and top bar so that custom composer can be done or custom top header etc..

🎨 UI Changes

N/A

🧪 Testing

Run the Compose sample and verify.

☑️Contributor Checklist

General

• I have signed the Stream CLA (required)
• [ x] 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)
• [ x] PR targets the develop branch
• PR is linked to the GitHub issue it resolves

Code & documentation

• [ x] 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
• [ x] 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