Add Inline Pre-Send URL Preview Support in Message Composer

Introduce native support for inline URL preview generation within CometChatMessageComposer in the Flutter UI Kit. Currently, link preview metadata is fetched and displayed only after the message has been successfully sent, whereas modern messaging applications such as WhatsApp and Microsoft Teams provide real-time preview generation while the user is composing the message. This feature should detect valid URLs as they are typed or pasted, fetch preview metadata before sending, and display a dynamic preview card inline above the composer. Supporting pre-send link previews would significantly improve user confidence, enhance the messaging experience, and align the UI Kit with modern chat application standards without requiring developers to build complex custom implementations.