Determine whether an issue or MRs task list is complete via API

internal customer since the Delivery and Quality teams will use it. See https://gitlab.com/gitlab-org/gitlab-ce/issues/51636#note_145207141

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Problem to solve

It should be possible to determine whether all tasks defined within an issue or merge request's description are completed

Further details

In the Web Interface, it's possible to tell if all tasks are checked off due to the task item counter. This cannot be determined through an API call

Proposal

• Expose the number of completed versus incomplete tasks in the issue and mr API payload.
• Expose the percentage of completed tasks in the payload

What does success look like, and how can we measure that?

(If no way to measure success, link to an issue that will implement a way to measure this)

Links / references