Inherit project approval rules for merge request creation

What does this MR do and why?

Contributes to #388140 (closed)

Problem

We don't create approval rules for merge request unless they are explicitly provided by the user.

It currently works for creating MR via UI, because frontend provides a list of approval rules in approval_rules_attributes param.

It doesn't work for merge_request API endpoint, because we don't populate this attribute when it's not provided.

Solution

The code is complicated enough and requires a refactoring. The least invacive approach is to generate approval_rules_attributes list if it was not provided.

That should ensure that MergeRequest::CreateService will inherit project approval rules even if the user didn't provide them.

How to set up and validate locally

1. See comment: #388140 (comment 1335979022)

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