Add site health checks for Ads module.

[benbowler]

Summary

Addresses issue:

• Create Ads Module Site Health Checks #8245

Relevant technical choices

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

site-kit-wp/includes/Modules/Ads.php

Lines 132 to 135 in 9a2dfa3

	/**
	* There is no dedicated tag for Ads module.
	*/
	public function register_tag() {}

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

• My code is tested and passes existing unit tests.
• My code has an appropriate set of unit tests which all pass.
• My code is backward-compatible with WordPress 5.2 and PHP 5.6.
• My code follows the WordPress coding standards.
• My code has proper inline documentation.
• I have added a QA Brief on the issue linked above.
• 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]

Build files for 9758b20 have been deleted.

[zutigrm]

Thanks @benbowler LGTM.

I synced this branch with develop since the tag implementation was needed to confirm the behaviour.

I also updated the QAB to describe all cases, and take new tag implementation into account, which now brings the inline comment used to detect that tag is placed by Site Kit.

Note to the Merge reviewer: There is a weird CI only test failure for php 5.6, it might be some glitch, couldn't replicate it locally, and the code change here is very small to affect it

[eugene-manuilov]

Thanks, @benbowler. A few comments from me. Please, take a look.

[eugene-manuilov]

This is not needed because it is already added to the class.

[eugene-manuilov]

Let's add a description for what is returned from this function to make it consistent with other method phpdocs.

[eugene-manuilov]

Let's also verify that the value is the same as we set in the settings.

[benbowler]

Thanks @eugene-manuilov, updates pushed.