Adds discovery mechanism for operator components.

[f41gh7]

I suggest to add remoteWrite, remoteRead label selectors, that will simplify installation and managing.

Abstract example:

apiVersion: operator.victoriametrics.com/v1beta1
kind: VMAgent
metadata:
  name: example-vmagent
 remoteWriteSelector:
    nameSpaceSelector: {}
    labelSelector: {}

This setting will build proper FQDN url for VMSingle and VMCluster components.

The same for VMAlert connections.

[hagen1778]

cc @Amper