Add "new chat" button option for header

[rebeccaalpert]

We want to add three things:

1. A button that can be optionally added to the header, with tests for new button
2. Add a header demo with button in header to https://www.patternfly.org/patternfly-ai/chatbot/ui#header-options if it doesn't break the styling - otherwise complain to Rebecca
3. Add icon to existing "new chat" button in chat history drawer

The icon is PenToSquare - I have a PR up to React to bump Font Awesome so we can pull this in: patternfly/patternfly-react#11905. You can use @patternfly/react-icons@6.3.0-prerelease.10 to get the icon (just update package.json file for local dev) if we haven't done a formal release yet.

Design is here: patternfly/patternfly-design-kit#796 (comment)

[kaylachumley]

related issue: patternfly/patternfly-design-kit#796