Move existing feature methods into Projects::BranchRule under conditional

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Once we have a basic model configured we need to be able to fetch the correct setting for the rule type. This includes

1. approval_project_rules
2. squash_option
3. external_status_checks

This needs to be behind a feature flag e.g.

      def approval_project_rules
        Feature.enabled?(:persisted_branch_rules)
          case rule_type
          when :default then protected_branch.approval_project_rules_with_unique_policies
          when :all_branches
            project.approval_rules.for_all_branches
          when :all_protected_branches
            project.approval_rules.for_all_protected_branches
          end
        else
          protected_branch.approval_project_rules_with_unique_policies
        end
      end

Where protected_branch.approval_project_rules_with_unique_policies is the original method from Projects::BranchRule. If no method existed for BranchRule before merging all rule types we return nil.