Add approval stats to merge requests API

Description

API response from "getting merge requests" does not include any approval statistics. Users have to call Merge Request Approvals API for each merge request to get this information.

Proposal

Enhance the following APIs to have approvals_required and approvals_left in the response:

• List merge requests
• List project merge requests
• Get single MR

Links / references

• https://docs.gitlab.com/ee/api/merge_requests.html
• ZD: https://gitlab.zendesk.com/agent/tickets/80955

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.

• Feature assurance
• Documentation
• Added to features.yml