Updates ApprovalMergeRequestRules with new enum for the `rule_type`
subtype of 'report_approver'.

Initially there is only 1 report_type ('security') however this can
later be expanded to include other rules tied to report_approvers;
i.e.  code_quality or license_management.

This also includes some shims to the ApprovalRuleLike logic ensuring
compatibility between the existing ApprovalProjectRule and
ApprovalMergeRequestRule types