Add support for S3 bucket sources

[stefanprodan]

This PR adds support for S3 bucket sources fix: #116

Changes:

• add Bucket kind to supported sources
• add a watcher to detect bucket revision changes

Example

Define a source that points to a S3 bucket hosted with Minio:

apiVersion: source.toolkit.fluxcd.io/v1alpha1
kind: Bucket
metadata:
  name: podinfo
spec:
  interval: 1m
  bucketName: podinfo
  endpoint:  minio.minio.svc.cluster.local:9000
  insecure: true
  secretRef:
    name: minio-credentials

Define a Kustomization that reconciles the Kubernetes manifests from the bucket on the cluster:

apiVersion: kustomize.toolkit.fluxcd.io/v1alpha1
kind: Kustomization
metadata:
  name: podinfo
spec:
  interval: 5m
  path: "./"
  prune: true
  sourceRef:
    kind: Bucket
    name: podinfo
  validation: client
  healthChecks:
    - kind: Deployment
      name: podinfo
      namespace: minio-test
  timeout: 2m

[hiddeco]

The 😈 works hard, but Stefan works harder 🛸