Request: Add support for rounded header corners [gridjs-react]

[luistv-swe]

Is your feature request related to a problem? Please describe.
(Using gridjs-react) I would like to have a sticky header with rounded corners, but the table rows are seen in the background of the sticky header because of the rounding. I'd prefer to have access to such an element through gridjs-react rather than having to access the DOM.

Describe the solution you'd like
I'd like to have an option to add a tr element into the thead element that we can style to match our header background and modify the z-index in order to cover up the table body showing behind the header. Example in React:

<thead className=''>
        {thContent}
	<tr className='gridjs-thead-rounded-corner-helper' />
</thead>

Describe alternatives you've considered
Any other solution that would allow for a sticky header with rounded corners without tbody content showing behind thead.

Additional context