Add site health checks for Ads module.

[benbowler]

Summary

Addresses issue:

• #8245

Relevant technical choices

In order to implement ModuleWithTag I had to implement a register_tag function:

https://github.com/google/site-kit-wp/blob/9a2dfa37f626b6ed9f96043a0b2b8cf4763bad58/includes/Modules/Ads.php#L132-L135

I also tested the regex used would capture the provided gtag config call.

This PR returns the Ads Conversion ID to the Site Health Checks but without the tag registered I was not able to see the tag registration check on this page as implemented.

PR Author Checklist

• [x] My code is tested and passes existing unit tests.
• [x] My code has an appropriate set of unit tests which all pass.
• [x] My code is backward-compatible with WordPress 5.2 and PHP 5.6.
• [x] My code follows the WordPress coding standards.
• [x] My code has proper inline documentation.
• [x] I have added a QA Brief on the issue linked above.
• [x] I have signed the Contributor License Agreement (see https://cla.developers.google.com/).

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Code Reviewer Checklist

• [ ] Run the code.
• [ ] Ensure the acceptance criteria are satisfied.
• [ ] Reassess the implementation with the IB.
• [ ] Ensure no unrelated changes are included.
• [ ] Ensure CI checks pass.
• [ ] Check Storybook where applicable.
• [ ] Ensure there is a QA Brief.

Merge Reviewer Checklist

• [ ] Ensure the PR has the correct target branch.
• [ ] Double-check that the PR is okay to be merged.
• [ ] Ensure the corresponding issue has a ZenHub release assigned.
• [ ] Add a changelog message to the issue.

[github-actions[bot]]

Build files for 9758b20f3902981a83d2c2831fa67f9c39cae6c4 have been deleted.

[benbowler]

Thanks @eugene-manuilov, updates pushed.