fix: update consumers when custom_id is also set

[GGabriele]

Right now decK only uses ID and Username for Consumers lookup, which is problematic because CustomID is also unique and modifying a Username causes a Consumer re-creation, which triggers a collision for the CustomID field.

This PR makes sure that CustomID is included in the lookup loop, so that such Consumers changes can be made in-place.

Solves https://github.com/Kong/deck/issues/698