API field for keeping track of all replicas for a dc

[0xmichalis]

Forked from #5875 (comment)

We need a field similar to the upstream deployment.status.replicas in order to 1) get closer to the upstream model and 2) have that field computed only in the controller where we already list all the deployments in every dc reconclilation and re-use elsewhere instead of computing it separately when hitting the dc scaling endpoint.

cc: @ironcladlou @smarterclayton @DirectXMan12

[0xmichalis]

Needed also for #6426