Add report_approver to approval_merge_request_rules

What does this MR do?

Adds report_approver (bool) and report_type (enum) to approval_merge_request_rules. This is the first of several MRs to satisfy Security approval in merge requests MVC https://gitlab.com/gitlab-org/gitlab-ee/issues/9928

In order to do so we must expand the functionality of the ApprovalMergeRequestRule model to support a new approval rule type and category.

There is no exclusivity logic to differentiate approval.report_approver? from approval.code_owner? as I don't believe these to be mutually exclusive (consider supporting a report_type syntax in the CODE_OWNERS file), although once we get deeper into the logic for #9928 (closed) it may be necessary to differentiate.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• [-] Merge request performance guidelines
• [-] Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team