feat(ButtonGroup): add segmented ButtonGroup to next entry

[damiengillesscality]

Summary

Adds a ButtonGroup component, exported from @scality/core-ui/next, that restyles its child Buttons into a single segmented control.

This generalises the local SegmentedControl/SegmentedGroup introduced in artesca-maestro#650 (the deployments-list "Sort by" / "Group by" controls) into a reusable core-ui component, so maestro can drop its bespoke styled.div/styled.button and consume this instead.

The API is inspired by MUI's ButtonGroup/ToggleButtonGroup but kept intentionally minimal and adapted to our use case.

API

Prop	Purpose
value + onChange	Optional single-select toggle. Each child carries a value; the matching child is highlighted (aria-pressed). Pass null for no selection.
orientation	'horizontal' (default) or 'vertical'.
children	core-ui Buttons. Without value/onChange the group is a plain visual cluster, each child keeping its own onClick.

import { Button, ButtonGroup } from '@scality/core-ui/next';

<ButtonGroup value={sortBy} onChange={handleSortChange} aria-label="Sort by">
  <Button value="name" label="Name" />
  <Button value="version" label="Version" />
</ButtonGroup>

Behaviour

• Joins child buttons into one bordered group, collapsing their individual radii/borders into shared hairline separators.
• Selected segment is highlighted using the existing theme tokens (backgroundLevel1, textPrimary, selectedActive) — same look as the original maestro control.
• null value renders no selection (supports maestro's clearable "Group by").
• Vertical groups render equal-width rows; horizontal cells keep their natural width.

Stories

Components/ButtonGroup: SortBy, GroupByClearable, Vertical, PlainGroup, Playground.

Tests

5 unit tests (rendering, aria-pressed selection, onChange payload, child onClick preservation, no selection props without onChange). eslint and tsc --noEmit clean.

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[bert-e]

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[damiengillesscality]

/approve

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