Implement StickyPinnedMessagesSubheader to showcase scrollToMessage.

[VelikovPetar]

🎯 Goal

Showcase of the scrollToMessage functionality:

• Adds a subheader in the MessagesActivity showing a rotating list of the channel pinned messages
• Tapping on the current pinned messages, focuses the message in the channel

🛠 Implementation details

• Implements a new StickyPinnedMessagesSubheader in the Compose sample app

🎨 UI Changes

Demo
demo.mp4

[github-actions]

SDK Size Comparison 📏

SDK	Before	After	Difference	Status
stream-chat-android-client	3.23 MB	3.23 MB	0.00 MB	🟢
stream-chat-android-offline	3.45 MB	3.45 MB	0.00 MB	🟢
stream-chat-android-ui-components	10.54 MB	10.54 MB	0.00 MB	🟢
stream-chat-android-compose	12.76 MB	12.76 MB	0.00 MB	🟢