feat: add support for custom CSS styles in copilot

[fgalind1]

This pull request allows for customization of copilot styles and allows for customization of styles of the copilot container in expanded or collapse mode.

for example:

window.mountChainlitWidget({
        chainlitServer: url,
        customCssUrl: '/public/copilot.css',
    }

and copilot.css:

#cl-shadow-root .copilot-container-expanded {
    height: min(900px,calc(100vh - 150px));
}

#cl-shadow-root .copilot-container-collapsed {
    width: min(600px,80vw);
}

other classes could be overwritten or customized as needed

[willydouhard]

lgtm!