feat: Support iframes in container media content

[avinashbot]

Description

We'd like to support automatically scaling the <iframe> element as a direct child of the media container. For youtube embeds and such.

Related links, issue #, if available: AWSUI-22100

How has this been tested?

There are screenshot tests for this styling on images already. Don't know if it's worth adding iframe-specific screenshot tests and scenarios for just width: 100%; height: 100%. iframes now have custom styling (removing the border), so added some permutations.

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 has no change and project coverage change: +0.12% 🎉

Comparison is base (fb4b027) 93.81% compared to head (03c34ee) 93.93%.
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1487      +/-   ##
==========================================
+ Coverage   93.81%   93.93%   +0.12%     
==========================================
  Files         640      642       +2     
  Lines       17273    17276       +3     
  Branches     5683     5682       -1     
==========================================
+ Hits        16204    16228      +24     
+ Misses        996      976      -20     
+ Partials       73       72       -1     

Files Changed	Coverage Δ
src/__a11y__/axe.ts	100.00% <ø> (ø)

... and 22 files with indirect coverage changes

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[just-boris]

Could you add an iframe example to dev page?

We cannot render external resources, but I think we can get an inline content like <iframe>it works</iframe>