Add generic browser extension action dispatch

[akirk]

Summary:

• add a generic Friends browser extension inline action dispatcher
• pass browser extension key/version context to friends_browser_extension_actions
• authenticate action requests with the existing browser API key and run handlers as that user

Verification:

• php -l includes/class-rest.php
• php -l tests/test-rest.php
• composer check-cs
• git diff --check

Note:

• PHPUnit could not run locally because /tmp/wordpress-tests-lib/includes/functions.php is missing.

[github-actions]

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