Create Projects::AllBranchesRules::MergeRequestApprovalSetting model

This will be used to mimic the Projects::BranchRules::MergeRequestApprovalSetting for All branches rule

We need to create an initializer that takes the project and makes it accessible to the instance

We will add logic to fetch the different values in another follow up

As part of this issue we should add logic to Projects::AllBranchesRule to instantiate this object e.g.

In CE

def merge_request_approval_settings
  nil
end

In EE

def merge_request_approval_settings
  Projects::BrnachRules::MergeRequestApprovalSetting.new(project: project)
end
strong_memoize :merge_request_approval_settings

Add tests to check that we can load the object correctly.