Add an option for a footer and a custom header in the ChatPopUp

[petia182]

We wanted to add a footer to the ChatPopUp, but we noticed that the component is not very flexible. There was no way to add the footer outside the component and style it in the correct position as the pop up chat has position: fixed styling.

So, I wanted to propose a small feature - adding an option for a footer text. This would be helpful for displaying a link to the Privacy Policy for example.
I added the footer inside the ChatPanel component, so it can be displayed in the ChatPopUp component as well and be in the correct position.

This PR is just a quick example, the footer can be converted into a separate component of course, for now, I just added it as a div inside the ChatPanel component.

The only thing I couldn't add is the styling for links inside the footer. In the example below I'm using react-markdown to render rich text and display a link. Ideally, the link should be styled in a different color.

[mdavish]

What do you think about making the footer prop accept string | ReactNode so that you can write your own JSX if need be? I imagine it'll be common to want <a/> tags in your footer.

[mdavish]

Or actually sorry, I didn't realize the footer accepted markdown. Might still be nice to accept a React Node though. Curious what others think.

[yen-tt]

thinking about it more, this wouldn't work because the header component needs to take in the onClose function below in order to control the display behavior of the chat popup/panel. We would need to take in a header component type and construct it inside.

[anguyen-yext2]

I have added support for the footer in this PR and added an item in the backlog for custom header support