Expand out user and group names in humanized security policy description

Currently the description for policies shows user IDs instead of their names.

Proposal

Expand this out to display the user and group names instead of their IDs.

• backend Add user_approvers and group_approvers as new fields into scan_result_policy_type. Types::UserType and Types::GroupType are already available.
• backend Add user_approvers and group_approvers attributes within the scope of policy_configuration.scan_result_policies.map in scan_result_policy_resolver. Their values can be fetched similarly to process_scan_result_policy_service.
• frontend Add userApprovers and groupApprovers with attributes equivalent to user_basic and in scan_result_policies.