[Vue] migrate widget-loader directive #18428

Description:

This PR is based off of #18423.

Changes:

• Migrate widgetloader directive.
• Allow sending custom headers in AjaxHelper.fetch

Review

• [ ] Functional review done
• [ ] Potential edge cases thought about (behavior of the code with strange input, with strange internal state or possible interactions with other Matomo subsystems)
• [ ] Usability review done (is anything maybe unclear or think about anything that would cause people to reach out to support)
• [ ] Security review done
• [ ] Wording review done
• [ ] Code review done
• [ ] Tests were added if useful/possible
• [ ] Reviewed for breaking changes
• [ ] Developer changelog updated if needed
• [ ] Documentation added if needed
• [ ] Existing documentation updated if needed

the interface worked as expected, but it seems like failed a test, not sure that is related. but it doesn't close the notification bar on the test I think.

 2) Notifications
       should still show persistent notifications on reload:
          the 'compare' command output could not be parsed, should be an integer, got: compare-im6.q16: image widths or heights differ `/home/travis/build/matomo-org/matomo/plugins/CoreHome/tests/UI/expected-screenshots/Notifications_reloaded.png' @ error/compare.c/CompareImageCommand/1000.
            Generated screenshot: /home/travis/build/matomo-org/matomo/tests/UI/processed-ui-screenshots/Notifications_reloaded.png
            Expected screenshot: /home/travis/build/matomo-org/matomo/plugins/CoreHome/tests/UI/expected-screenshots/Notifications_reloaded.png

            Url to reproduce: http://localhost/tests/PHPUnit/proxy/?module=CoreAdminHome&action=home&idSite=1&period=day&date=yesterday
            No captured console logs

UI tests should be passing for this PR

there is a test that failed, I think it's related to this. Maybe need to update the tests.