fix: Restore focus to custom popover triggers

[gethinwebster]

Description

Ensure that if a custom popover trigger is used, the focus is returned
to it when the popover is dismissed.

Related links, issue #, if available: AWSUI-21509

How has this been tested?

New integ tests

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: 100.00% and no project coverage change.

Comparison is base (c8425e8) 93.46% compared to head (9394858) 93.46%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1352   +/-   ##
=======================================
  Coverage   93.46%   93.46%           
=======================================
  Files         623      623           
  Lines       16733    16736    +3     
  Branches     5527     5530    +3     
=======================================
+ Hits        15639    15642    +3     
  Misses       1021     1021           
  Partials       73       73           

Impacted Files	Coverage Δ
src/popover/internal.tsx	93.33% <100.00%> (+0.35%)	⬆️

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[rubencarvalho]

LGTM!