Switch Idea Hub module REST route for updating idea states to actually use Idea Hub API

Broken out from #3518: Since the Idea Hub API route to update an idea’s state is still not working correctly, the POST:update-idea-state REST endpoint should be connected to the actual API separately, in this issue.

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Do not alter or remove anything below. The following sections will be managed by moderators only.

Acceptance criteria

• The POST:update-idea-state endpoint should use https://github.com/googleapis/google-api-php-client-services/blob/master/src/Google/Service/Ideahub/Resource/PlatformsPropertiesIdeaStates.php, passing platform and property similar to the other endpoints (see #3518) and the received idea name as the “ideaState” placeholder in the path. The data point should expect a single variable ideaState which needs to be an object / associative array with name (idea name string, also used in the path as mentioned right above), saved (boolean) and dismissed (boolean) fields.
• In other words, with this issue, saving/unsaving and dismissing an idea should actually begin to work / result in the idea no longer showing up in the new/saved tab.

Implementation Brief

• Verify that the approach in #3767 works as intended when the corresponding Idea Hub API starts to work correctly. If it doesn’t work as intended (see AC) then make necessary changes and send PR for code review

Test Coverage

• N/A

Visual Regression Changes

• N/A

QA Brief

• Activate Idea Hub module
• Set the custom site URL setting in the Site Kit developer settings to https://www.elasticpress.io/
• Go to the dashboard and verify that you see new ideas
• Click on the pin icon to save the idea. Verify that the saved idea has been moved to the “Saved” ideas tab.
• Refresh the page and verify that the saved ideas list contains the recently saved idea and the new ideas list doesn’t contain it anymore.
• Now unsave the idea and verify that the idea has been moved to the ideas list (at the end). Plus after refreshing the page you see it in the new ideas list and don’t see it in the saved ideas anymore.
  • NOTE: While the above is what should happen here, there appears to be currently a bug with the API that prevents the first sentence above from working. In other words, the (for now broken) accepted functionality is: When unsaving, at first the idea won’t move back to the “New” tab. But it should at least have been moved there once you refresh the page. This likely needs to be fixed at the API level (also see https://github.com/google/site-kit-wp/pull/3767#discussion_r685573564).
• Dismiss an idea and verify that the idea doesn’t re-appear after refreshing the dashboard page.

Changelog entry

• Implement functionality for saving and dismissing an Idea Hub idea.

[eugene-manuilov]

@felixarntz QAB is added.

[felixarntz]

@eugene-manuilov Can you also add a QA Brief here? It should include some basic testing of saving, unsaving, and dismissing, since that’s now enabled by this API endpoint.