refactor: use additive diffing (no side effects)

[acpana]

Mostly from Walter:

Return Values Directly: Go's multiple return values are a powerful feature. Prefer returning values directly rather than using "out parameters" (passing pointers to be modified). If you do not need values from an input parameter then it should probably be a return value.

The ask here is to actually report a diff in places like this as opposed to adding fields to the existing diff passed in as a parameter

k8s-config-connector/pkg/controller/direct/sql/sqlinstance_equality.go

Line 25 in e364c69

func InstancesMatch(desired *api.DatabaseInstance, actual *api.DatabaseInstance, diff *structuredreporting.Diff) bool {

Originally posted by @acpana in #5152 (comment)