Implement sharding for Flux controllers

[stefanprodan]

This PR allows configuring sharding and horizontal scaling for the Flux controllers.

Sharding configuration

The .spec.sharding field is optional and specifies the sharding configuration for the Flux controllers.

The .spec.sharding.key field is optional and specifies the sharding key label to use for the Flux controllers.
By default, the key is set to sharding.fluxcd.io/key.

The .spec.sharding.shards field is required and specifies the list of sharding values to use for the Flux controllers.

Example:

apiVersion: fluxcd.controlplane.io/v1
kind: FluxInstance
metadata:
  name: flux
  namespace: flux-system
spec:
  distribution:
    version: "2.3.x"
    registry: "ghcr.io/fluxcd"
    artifact: "oci://ghcr.io/controlplaneio-fluxcd/flux-operator-manifests"
  sharding:
    key: "sharding.fluxcd.io/key"
    shards:
      - "shard1"
      - "shard2"

For each shard, the operator will create a separate set of controllers, e.g.:

$ kubectl -n flux-system get deployments -l app.kubernetes.io/part-of=flux
NAME
source-controller
source-controller-shard1
source-controller-shard2
kustomize-controller
kustomize-controller-shard1
kustomize-controller-shard2
helm-controller
helm-controller-shard1
helm-controller-shard2

Note that only the source-controller, kustomize-controller and helm-controller controllers
support sharding.

To assign a resource to a specific shard, add the sharding.fluxcd.io/key label with the shard value,
e.g.: sharding.fluxcd.io/key: shard1.