Add support for sections to Select (#34012)

Updates the Select component to accept a set of sections with options as opposed to just a flat list of options.

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Signed-off-by: James Brooks <jamesbrooks@spotify.com>

.changeset/fancy-parents-sit.md

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+---
+'@backstage/ui': patch
+---
+
+Added support for grouping options into sections in the Select component. You can now pass section objects with a `title` and a nested `options` array alongside (or instead of) regular options to render grouped dropdowns with section headers.
+
+**Affected components:** Select