Should be able to disable the `New chat` button

[karthikjeeyar]

onNewChat boolean prop in ChatbotConversationHistoryNavProps allows to either show/hide the New chat button on the conversation history panel, the ask from RHDH is to make it disabled instead of hiding the new chat button when the user is already on a new chat session.

Current behaviour in RHDH:

In existing chat session	In new chat session

Proposed solution:

Create a new optional prop to disable the new chat button rather than only relying on onNewChat prop to show/hide the button.