feat: editable variable for modify functionality (#655)

## Problem
The current chat interface lacks support for editing commands/messages
after they have been sent. This limits user experience when users need
to modify their chat prompts or correct mistakes without starting a new
conversation thread.

## Solution
Added support for message editing by extending the chat type
definitions:
add editable property to ChatMessage and editedText to ButtonClickParams

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## License

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license.

Author: qjhuangAWS

types/chat.ts

@@ -170,6 +170,7 @@ export interface ChatMessage {
     body?: string
     messageId?: string
     canBeVoted?: boolean // requires messageId to be filled to show vote thumbs
+    editable?: boolean
     relatedContent?: {
         title?: string
         content: SourceLink[]