Add reconciler sharding capability based on label selector

[makkes]

The new --watch-label-selector lets operators provide a label to the controller manager which in turn uses it to reconcile only those resources (ImageRepositories and ImagePolicies) that match the given label.

This way operators can deploy multiple instances of IRC, each reconciling a distinct set of resources based on their labels and effectively scale IRC horizontally.

⚠️ WARNING

If sharding is enabled, all ImagePolicy resources can only refer to those ImageRepository resources that are captured by the exact same label selector as the ImagePolicies, otherwise the ImagePolicies will not become ready.

Example

Given two Deployments with different shard keys:

1. Deployment image-reflector-controller-1 with --watch-label-selector=sharding.fluxcd.io/shard=shard1
2. Deployment image-reflector-controller-2 with --watch-label-selector=sharding.fluxcd.io/shard=shard2

An ImageRepository like this:

apiVersion: image.toolkit.fluxcd.io/v1beta2
kind: ImageRepository
metadata:
  labels:
    sharding.fluxcd.io/shard: shard1
[...]

An ImagePolicy like this:

apiVersion: image.toolkit.fluxcd.io/v1beta2
kind: ImagePolicy
metadata:
  labels:
    sharding.fluxcd.io/shard: shard1
[...]
spec:
  imageRepositoryRef:
    name: podinfo
[...]

The ImagePolicy will successfully be reconciled.

If the ImagePolicy would have the label sharding.fluxcd.io/shard: shard2 reconciliation would fail because it would be referring to an ImageRepository from a different shard.

closes #364

[stefanprodan]

For IRC things are a more complicated, can you please explain in the PR description that the same shard hey must be used for all policies referring to a repo including that repo. Also writing down an example like I did in fluxcd/pkg#525 would be great.

[stefanprodan]

The github.com/fluxcd/pkg/apis/meta v0.19.1 should be v1.0.0

[stefanprodan]

LGTM

Thanks @makkes