Add support for sparse checkout to GitRepository API

[stefanprodan]

With the implementation of sparse checkout in fluxcd/pkg/git we can add this as an opt-in feature to the GitRepository API.

To enable sparse checkout, users will have to specify a list of directories under .spec.sparseCheckout e.g.

apiVersion: source.toolkit.fluxcd.io/v1
kind: GitRepository
metadata:
  name: podinfo
  namespace: default
spec:
  interval: 5m
  url: https://github.com/stefanprodan/podinfo
  ref:
    branch: master
  sparseCheckout:
    - ./charts
    - ./kustomize