chore: Allow dynamic deferred loading of internationalization

[avinashbot]

Description

Partial implementation of dynamically loaded internationalization. This lets us more readily embed the I18nProvider into performance-sensitive parts of the code (like the app layout toolbar) without worrying so much about added bundle size of the intl library, intl-messageformat (34.3 kB, 9.83 kB gzip) and the locale files (465kB, 68kB gzip). This also avoids dealing with any additional build-time considerations (intl-messageformat raises our bare minimum TypeScript version requirement, which we don't document publicly, though we do call out the raised minimum in our documentation).

The added cost is one asynchronously triggered full-application rerender.

components/LocalI18nProvider  <- The application may provide this already (this is fine)
↳ components/AppLayout
  ↳ components/AppLayout/skeleton
    ↳ components/RemoteI18nProvider <-------+
      ↳ application code                    | 
      ↳ widgetized/AppLayout/toolbar        | --- Once messages and formatter are loaded,
      ↳ widgetized/AppLayout/drawers        |     the content is re-rendered.
    ↳ widgetized/loadRemoteI18nMessages() --+

"Dynamic loading" of the internationalization bundle isn't all that complicated, it's just a runtime import of I18nFormatter (which this PR refactors out) preloaded with the appropriate locale file (which are already exposed under /messages/all.<locale>.js).

---

Possible optimizations (either later in this PR or after release):

• Wrapping the app rerender in a React transition: For React versions that support it, it avoids our rerender from blocking user actions and affecting web vitals.
• Removing ICU parser from the remote bundle (reference): If we can safely guarantee that the remote provider only accepts and handles pre-parsed AST-only strings, we can remove the parser from the dynamically loaded bundle and reduce the dynamic bundle's file size by 26.34 kB (7.1 kB gzip). But this is a non-blocking network request, so it's not high priority. See CR-253557644.

Related links, issue #, if available: AWSUI-61508

How has this been tested?

Existing tests for locally (i.e. application) provided strings, additional tests for remote inclusion of logic will follow.

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

❌ Patch coverage is 97.87234% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.18%. Comparing base (9fc78ce) to head (a270b41).

Files with missing lines	Patch %	Lines
src/i18n/providers/remote-provider.tsx	90.47%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4237      +/-   ##
==========================================
- Coverage   97.19%   97.18%   -0.01%     
==========================================
  Files         886      889       +3     
  Lines       26021    26049      +28     
  Branches     9433     9438       +5     
==========================================
+ Hits        25291    25317      +26     
+ Misses        724      685      -39     
- Partials        6       47      +41     

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