[EuiDataGrid] Enhance renderCustomGridBody to better support consumer virtualization

packages/eui/changelogs/upcoming/8028.md

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+**Breaking changes**
+
+- EuiDataGrid's custom grid body (rendered via `renderCustomGridBody`) no longer automatically renders the column header row or footer rows. It instead now passes the `headerRow` and `footerRow` React elements, which require manual rendering.
+  - This change was made to allow consumers to sync header/footer rows with their own custom virtualization libraries.
+  - To facilitate this, a `gridWidth` prop is now also passed to custom grid body renderers.