Create the backend needed for manually completing tasks

[leonidasmi]

Context

Summary

This PR can be summarized in the following changelog entry:

• Creates the backend needed for manually completing tasks.

Relevant technical choices:

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• With a fresh task list (aka, no task being completed), perform a POST request to http://example.com/wp-json/yoast/v1/manually_complete_task with the following body:

{
    "options": {
        "task_id": "delete-hello-world",
        "completed": true
    }
}

where task_id is the ID of a task (you can check the IDs of tasks if you have the network tab open when you first load the task list and check the get_tasks response)

• Do a GET request to http://example.com/wp-json/yoast/v1/get_tasks and confirm that the task that you used its ID has isCompleted equal to true
• Now perform a POST request to http://example.com/wp-json/yoast/v1/manually_complete_task to revert that previous manual completion, with the following body:

{
    "options": {
        "task_id": "delete-hello-world",
        "completed": false
    }
}

where task_id is the ID of a task

• Do a GET request to http://example.com/wp-json/yoast/v1/get_tasks and confirm that the task that you used its ID has isCompleted equal to false again.
• Do the same test with at least one Premium task too, eg. with organization-schema.

Relevant test scenarios

• Changes should be tested with the browser console open
• Changes should be tested on different posts/pages/taxonomies/custom post types/custom taxonomies
• Changes should be tested on different editors (Default Block/Gutenberg/Classic/Elementor/other)
• Changes should be tested on different browsers
• Changes should be tested on multisite

Test instructions for QA when the code is in the RC

• QA should use the same steps as above.

Impact check

This PR affects the following parts of the plugin, which may require extra testing:

Other environments

• This PR also affects Shopify. I have added a changelog entry starting with [shopify-seo], added test instructions for Shopify and attached the Shopify label to this PR.
• This PR also affects Yoast SEO for Google Docs. I have added a changelog entry starting with [yoast-doc-extension], added test instructions for Yoast SEO for Google Docs and attached the Google Docs Add-on label to this PR.

Documentation

• I have written documentation for this change. For example, comments in the Relevant technical choices, comments in the code, documentation on Confluence / shared Google Drive / Yoast developer portal, or other.

Quality assurance

• I have tested this code to the best of my abilities.
• During testing, I had activated all plugins that Yoast SEO provides integrations for.
• I have added unit tests to verify the code works as intended.
• If any part of the code is behind a feature flag, my test instructions also cover cases where the feature flag is switched off.
• I have written this PR in accordance with my team's definition of done.
• I have checked that the base branch is correctly set.
• I have run grunt build:images and commited the results, if my PR introduces new images or SVGs.

Innovation

• No innovation project is applicable for this PR.
• This PR falls under an innovation project. I have attached the innovation label.
• I have added my hours to the WBSO document.

Fixes #