[feat] Programmatically submit a user prompt (and optional attachments)

I think it'd be great if the chat exposed a controller for more flexible handling of the following scenarios:

* Attaching files (For example ClipboardData) - right now this is possible with a custom messageSender, but the files aren't visible in the chat before submission
* Call sendMessage programmatically. For example there might be predefined prompts, which should be executed on click.
* Manage focus of the Chats Input. Especially on Desktop there are Scenarios, where manual focus management might be required (Also the current approach behaves kinda weird)