Copilot chat: Remove attachment from chat session (previous turns)

**Use case:**
If a file from a previous turn is a sensitive file or a file that is not relevant to the context, there should be the option to remove it from the chat session history / context, whilst preserving all of the rest of the chat session history / context.

**Note:** Upon removing, it could have a reference where the attachment had showed previously. E.g., "[filename] was removed from history / context."