MSD: Add PageHeader.ActionMenu to provide mobile styles.

[StevenDufresne]

Alternative to #107085.
Related: #107003

Proposed Changes

Adds a PageHeader.ActionMenu wrapper component to specifically handle DropdownMenu on small screens.

Testing Instructions

Visit the following pages on mobile/desktop, expect the dropdown menu to be right-aligned on mobile and right beside the buttons to the right on desktop.

• /v2/sites/:site
• /v2/domains/:domain/dns
• /v2/me/billing/purchases/:purchaseId

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you tested accessibility for your changes? Ensure the feature remains usable with various user agents (e.g., browsers), interfaces (e.g., keyboard navigation), and assistive technologies (e.g., screen readers) (PCYsg-S3g-p2).
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
  • For UI changes, have we tested the change in various languages (for example, ES, PT, FR, or DE)? The length of text and words vary significantly between languages.
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?

[github-actions]

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[arthur791004]

I meant to something like below but the current way also looks great 👍

const ActionMenu = ( props: ComponentProps< DropdownMenu > ) => {
  return <DropdownMenu {...props} style={ { marginInlineStart: 'auto' } } />;
}