Connectors: add application password authentication

[jorgefilipecosta]

What?

• Adds an application_password connector authentication method with generated username and application-password setting names.
• Registers both settings, masks only the application password in REST responses, and exposes non-secret connection metadata to the admin UI.
• Adds registry, Settings API, REST masking, and script-module tests.

Testing

• php ./vendor/bin/phpunit --group connectors
• Targeted PHPCS for the changed Core files
• Verified end to end with Connectors: add application password settings UI gutenberg#79403 by pointing WP_ENV_CORE at this checkout

Companion UI: WordPress/gutenberg#79403
Trac: https://core.trac.wordpress.org/ticket/64850

[github-actions]

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Props jorgefilipecosta, jorbin.

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[aaronjorbin]

Since this and the password one are only used when application_password is set as the authentication method, I think it might be helpful to either call this out in the docs or do a _doing_it_wrong if this or application_password_setting_name is passed in when application_password is not the authenciation method (or maybe both)

[jorgefilipecosta]

Good point, added a _doing_it_wrong validation.